Ulasan tentang Susunan Nanotube Titania yang Terorganisasi Secara Elektrokimia:Sintesis, Modifikasi, dan Aplikasi Biomedis

Sintesis TiO₂ Array Nanotube dengan Anodisasi Elektrokimia

Dalam beberapa tahun terakhir, sementara berbagai bentuk titanium dioksida berstrukturnano termasuk nanorods, nanopartikel, kawat nano, dan nanotube telah berhasil dikembangkan [29,30,31], nanotube telah menarik minat yang meningkat untuk aplikasi teknologi karena struktur rakitan unik dengan area antarmuka yang besar dan pengontrolan ukuran dan bentuk yang nyaman, yang dapat diterapkan pada aplikasi yang bergantung pada luas permukaan sebagai kandidat yang lebih baik. Sejumlah ulasan yang sangat baik [1, 2, 15, 32,33,34] tersedia untuk menangani fitur TiO₂ nanomaterial dikategorikan dengan metode sintetis yang berbeda. Anodisasi elektrokimia terbukti menjadi salah satu metode yang paling efektif untuk mendapatkan nanotube titania sebagai teknologi yang relatif sederhana yang dapat diotomatisasi dengan mudah. Kami akan menentukan teknik utama untuk membuat TiO anodik₂ nanotube di bawah.

TiO Anodik yang diatur sendiri₂ Array Nanotube

Seperti dipelajari secara ekstensif, lapisan nanotube titania dapat dibentuk di bawah kondisi lingkungan tertentu. Perangkat oksidasi terdiri dari tiga bagian:(I) sistem tiga elektroda dengan foil Ti yang disiapkan sebagai elektroda kerja yang didegradasi dengan sonikasi berurutan dalam aseton, etanol, dan air deionisasi, platina sebagai elektroda lawan dan biasanya Ag/AgCl sebagai elektroda referensi (Gbr. 2a), sedangkan elektroda pH terkadang ditambahkan untuk mendapatkan konsentrasi akhir F ⁻ dan HF [35] atau sistem dua elektroda sederhana lainnya yang terdiri dari Ti foil sebagai anoda dan elektroda logam inert sebagai katoda (Gbr. 2b) [36]; (II) umumnya, ion fluorida, ion klorida, ion kromium, ion bromida, atau perklorat yang mengandung elektrolit; dan (III) catu daya DC. Ada dua fitur utama yang dipengaruhi oleh kondisi anodisasi formasi yang mempengaruhi aplikasi yang menjanjikan dari titania nanotube:(I) geometri:ukuran, bentuk, tingkat keteraturan, fase mengkristal, dll. dan (II) sifat kimia, fisik, dan biomedis. Dengan kata lain, melalui pengontrolan parameter anodisasi elektrokimia (potensial yang diterapkan, durasi anodisasi, sistem elektrolit termasuk konsentrasi ion fluor, dan air dalam elektrolit, suhu elektrolit, pH elektrolit, dll. yang akan dibahas lebih detail di bagian “Sintesis TiO2 Nanotube Arrays oleh Anodisasi Elektrokimia”), seseorang dapat membuat struktur nano titania yang berbeda seperti oksida kompak datar [1], lapisan berpori [1, 36], TiO₂ yang tidak teratur lapisan nanotube tumbuh dalam bundel [37], atau akhirnya TiO reguler yang sangat terorganisir₂ nanotube atau lapisan nanotubular canggih:tabung bercabang [38], struktur seperti bambu [38, 39], berdinding ganda [40], nanolace [38], atau lapisan ganda [39] di mana properti dapat ditemukan secara berbeda. Gambar 3 dan 4 menampilkan gambar mikroskop elektron pemindaian emisi lapangan (FE-SEM) dari contoh tipikal TiO₂ tersebut morfologi nanotube.

Pengaturan skema. a Gambar ilustrasi sistem tiga elektroda dengan foil Ti yang disiapkan sebagai elektroda kerja, platina sebagai elektroda lawan, dan biasanya Ag/AgCl sebagai elektroda referensi, sedangkan elektroda pH sebagai pH meter. Direproduksi dari ref. [35]. b Gambar ilustrasi sistem dua elektroda sederhana yang terdiri dari foil Ti sebagai anoda dan elektroda logam inert sebagai katoda. Anodisasi menyebabkan lapisan oksida anodized yang berbeda dalam kondisi yang berbeda. Dalam kebanyakan elektrolit netral dan asam, titania kompak dapat dibentuk. Tetapi jika elektrolit fluorida encer digunakan, lapisan oksida nanotubular/nanopori akan langsung menempel pada permukaan logam. Direproduksi dari ref. [36]

Gambar SEM dari TiO yang dianodisasi₂ lapisan nanotube dengan proses anodisasi Ti yang berbeda. a TiO yang sangat teratur₂ nanotube (dalam tampilan atas dan samping) diperoleh dalam sistem elektrolit organik, dengan lesung permukaan yang dipesan sendiri (kanan) yang sebenarnya adalah permukaan logam ketika lapisan tabung dilepas. Direproduksi dari ref. [1]. b TiO yang tidak teratur₂ nanotube tumbuh di patch pada area permukaan dan menyatu bersama-sama untuk bundel dalam klorida yang mengandung elektrolit dengan teknik anodisasi ultrafast yang dikenal sebagai anodisasi cepat-breakdown (RBA). Direproduksi dari ref. [1] dan [37]

Gambar SEM TiO tingkat lanjut₂ morfologi nanotube. a TiO diperkuat tipe bambu₂ nanotube dibuat di bawah kondisi tegangan bolak-balik (AV) tertentu dalam etilen glikol yang terdiri dari 0,2 mol/L HF, dengan urutan 1 menit pada 120 V dan 5 menit pada 40 V. Direproduksi dari ref. [38]. b Transisi dari TiO halus ke bambu₂ nanotube dapat diinduksi dengan anodisasi dengan penambahan air terkontrol (kadar air:1 hingga 8%) menjadi 0,135 M NH₄ Elektrolit F/etilen glikol

direproduksi dari ref. [39]. c Struktur nanolace 2D diperoleh di bawah siklus tegangan yang dilakukan untuk jangka waktu yang lama dalam elektrolit yang mengandung fluorida, dengan urutan 50 detik pada 120 V dan 600 detik pada 0 V. Direproduksi dari ref. [38]. d TiO berdinding ganda₂ nanotube ditumbuhkan dengan anodisasi Ti dalam elektrolit yang mengandung fluorida etilen glikol pada 120 V setelah anil pada 500 °C dengan laju pemanasan 1 °C s ⁻¹ . Direproduksi dari ref. [40]. e Tabung nano bercabang dapat diamati dengan loncatan tegangan, pertama pada 120 V (6 jam) dan kemudian pada 40 V (2 jam). Direproduksi dari ref. [38]. f Nanotube lapisan ganda dengan diameter tabung yang sama atau dua berbeda dapat dilihat. Direproduksi dari ref. [38]

(Saat ini, TiO₂ susunan nanotube dengan diameter tabung mulai dari 10 hingga 500 nm, ketebalan lapisan mulai dari beberapa ratus nanometer hingga 1000 m, dan ketebalan dinding berkisar antara 2 hingga 80 nm dapat diperoleh [15, 41].)

Itu dua dekade yang lalu ketika Masuda dan Fukuda untuk pertama kalinya melaporkan alumina berpori yang sangat teratur melalui penyesuaian kondisi anodisasi ke optimum [42]. Kemudian, para peneliti menghabiskan upaya mereka untuk membuat struktur yang terorganisir serupa juga untuk TiO₂ lapisan nanotube. Dan ada tiga faktor penting yang mempengaruhi derajat keteraturan pada TiO anodik₂ nanotube array (sesuai dengan poligon di lapisan dan standar deviasi diameter tabung):substrat Ti, tegangan yang diterapkan, dan anodisasi berulang [33, 43]. Jelas bahwa kekurangan yang lebih sedikit dalam pengaturan dapat diperoleh untuk bahan dengan kemurnian tinggi pada tegangan setinggi mungkin di bawah kerusakan dielektrik [33] dan TiO yang dipesan sendiri heksagonal idealnya₂ nanotube seperti yang ditunjukkan pada Gambar. 5 dapat ditingkatkan secara signifikan dengan pertumbuhan tabung sekunder [43]. Sofa dkk. menunjukkan pengotor sangat mempengaruhi dimensi yang berbeda yang dihasilkan dan pemesanan nanotube setelah anodisasi kedua [44]. Selain itu, orientasi kristalografi dari butiran substrat Ti telah terungkap sebagai efek penting dalam karakteristik pertumbuhan TiO₂ array nanotube dengan difraksi hamburan balik elektron (EBSD). Leonardi dkk. menemukan bahwa nanotube hanya dapat diamati dengan orientasi yang memungkinkan oksida logam katup terbentuk pada butiran yang memungkinkan penetrasi ion fluorida melalui film oksidasi di mana 1 M (NH₄ )H₂ PO₃ +0,5 % berat NH₄ F digunakan sebagai elektrolit [45]. Demikian pula, Macak dan rekan kerja melaporkan bahwa tidak ada pertumbuhan nanotube pada biji-bijian yang terhambat dalam elektrolit berbasis etilena glikol yang banyak digunakan dibandingkan dengan kasus penggunaan elektrolit berair, seperti yang diketahui dari literatur terakhir [46]. Pada lembaran Ti yang dipoles, butiran dengan orientasi [0 0 0 1] atau mendekati ini ternyata menjadi butiran yang ideal dan memanfaatkan kristal tunggal Ti dengan orientasi ideal akan menjadi kemajuan besar untuk mendapatkan susunan tabung nano yang paling seragam [46] .

Gambar SEM TiO₂ nanotube. Nanotube dibentuk dalam elektrolit etilen glikol yang mengandung 0,27 M NH₄ F dengan anodisasi berulang Ti. Penampang melintang diambil di bagian atas lapisan, di tengah, dan di bagian bawah lapisan. Direproduksi dari ref. [43]

Namun demikian, masih ada beberapa cacat yang mempengaruhi tingkat pesanan. Akhir-akhir ini, telah diperluas lebih lanjut dengan nanoimprinting seragam Ti. Kondo dkk. menemukan fabrikasi throughput TiO anodik yang dipesan secara ideal₂ dengan nanoimprinting permukaan Ti atau spesimen dua lapis dengan lapisan Al di bagian atas dan lapisan Ti di bagian bawah menggunakan cetakan Ni dengan cembung teratur. Dan TiO₂ lapisan dapat dihasilkan dengan cara yang lebih teratur di mana cekungan dangkal dari pola pra-tekstur bertindak sebagai situs inisiasi oleh anodisasi berikutnya di NH₄ larutan F etilen glikol [47, 48]. Mengikuti dari dekat, Sopha et al. terlebih dahulu melapisi lapisan pelindung TiN pada substrat Ti yang dibuat dengan deposisi lapisan atom (ALD) sebelum pra-tekstur dilakukan dengan sinar ion terfokus (FIB) dan selanjutnya anodisasi menggunakan elektrolit etilen glikol untuk menghasilkan lapisan nanotube yang tersusun sempurna secara heksagonal dengan ketebalan dari 2 m, yang dapat membatasi nanotube hanya untuk tumbuh di situs inisiasi yang diberikan dan memperpanjang waktu anodisasi tanpa cacat [49].

Mekanisme Pembentukan TiO Anodik₂ Tabung nano

Teknologi oksidasi anodik dan penelitian mekanisme pembentukan TiO anodik₂ nanotube telah menarik perhatian luas untuk waktu yang lama dari beragam disiplin ilmu. Mekanisme penelitian Diggle dilaporkan pada tahun 1969 tentang film oksida anodik kompak dan oksida anodik berpori [50] sekarang masih memainkan peran pemandu yang sangat penting. Sejumlah besar karya terbaru menunjukkan bahwa transisi dari pori-pori ke tabung bersifat bertahap [1, 27, 36]; namun, model dan alasan yang sepenuhnya teoretis tidak diberikan.

Pembubaran berbantuan lapangan konvensional (FAD) adalah teori yang paling dapat diterima [1, 33, 51]. Yaitu pada proses anodisasi elektrokimia, TiO₂ susunan nanotube dibentuk oleh pengaturan sendiri titania karena tiga prosedur yang relatif independen:oksidasi elektrokimia Ti menjadi TiO₂ , pelarutan TiO yang diinduksi medan listrik₂ , dan pelarutan kimia TiO yang diinduksi ion fluor₂ , mencapai keseimbangan yang halus. Sebagai kurva waktu arus karakteristik yang ditunjukkan pada Gambar. 6 untuk elektrolit yang mengandung fluorida yang mengarah pada pembentukan nanotube [51] dan gambar tipikal yang dapat membantu untuk menggambarkan proses pembentukan secara skematis [33] yang ditunjukkan pada Gambar. 7, transien dapat dibagi menjadi tiga tahap yang berbeda:(I) Pada bagian pertama, ada peluruhan saat ini, yang disebabkan oleh oksida penghalang yang baru terbentuk, ketika dua proses utama, migrasi ke dalam O ²⁻ ion menuju antarmuka logam/oksida dan migrasi keluar Ti ⁴⁺ ion menuju antarmuka oksida/elektrolit, mencapai keseimbangan. (II) Pada bagian kedua, arus mulai naik lagi dengan jeda waktu yang disebabkan oleh meningkatnya luas permukaan anoda. Semakin pendek lag, semakin tinggi konsentrasi fluorida karena pembubaran TiO yang diinduksi fluorida₂ , dan pori-pori mulai terbentuk secara acak yang selanjutnya menjadi awal pembentukan TiO₂ nanotube. (III) Kemudian, arus mencapai keadaan tunak, ketika laju pertumbuhan pori pada antarmuka oksida logam dan laju disolusi terinduksi dari TiO yang terbentuk₂ pada antarmuka luar mencapai situasi keseimbangan. Dengan demikian, tabung akhir menjadi semakin berbentuk v, yaitu, bagian atas tabung memiliki dinding yang jauh lebih tipis daripada bagian bawahnya di mana tabung tertutup rapat. Gradien pada ketebalan dinding tabung pada Gambar 5 dapat dianggap berasal dari waktu pemaparan dan konsentrasi yang berbeda terhadap elektrolit di sepanjang tabung [43].

Kurva waktu arus khas di bawah tegangan konstan dalam elektrolit yang mengandung fluorida. Transien dapat dibagi menjadi tiga wilayah yang berbeda (I –III ). (Saya ) Pada bagian pertama, ada peluruhan arus yang tajam. (II ) Pada bagian kedua, arus mulai naik lagi dengan jeda waktu. (III ) Di bagian ketiga, arus mencapai kondisi tunak yang direproduksi dari ref. [51]

Proses pembentukan TiO₂ array nanotube. Pembentukan TiO₂ array nanotube dapat dibagi menjadi tiga tahap morfologi yang berbeda (I –III ). (Saya ) Sebuah oksida penghalang terbentuk. (II ) Permukaan diaktifkan secara lokal dan pori-pori mulai tumbuh secara acak. (III ) Lapisan nanotube yang terorganisir sendiri terbentuk direproduksi dari ref. [33]

Namun, teori ini tidak dapat menjelaskan fenomena pemisahan ke dalam tabung, karena bertentangan dengan struktur nanopori yang belum jelas, dan Fahim et al. mengamati bahwa di bawah tegangan yang sesuai adalah mungkin untuk mendapatkan nanotube titania dalam larutan asam sulfat tanpa ion fluorida, dalam hal ini, kurva I-t menyerupai yang baru saja kita bahas di atas [52]. Seperti yang ditunjukkan Houser dan Hebert, mekanisme pertumbuhan belum dikembangkan untuk menjelaskan hubungan kuantitatif antara proses membran berpori titania dan kurva I-t [53]. Karena interpretasinya kurang meyakinkan, muncul poin-poin baru tentang mekanisme tersebut belakangan ini seperti model aliran viskos dan model pertumbuhan dua arus. Berkenaan dengan mekanisme ini, tinjauan [51] menunjukkan banyak keterbatasan untuk teori disolusi berbantuan lapangan tradisional dan membuat beberapa penjelasan tentang kemajuan terbaru dan signifikansi penelitian pada model aliran kental dan model pertumbuhan dua arus.

Pengaruh Kondisi Anodisasi yang Mempengaruhi Geometri dan Properti

Komposisi dan konsentrasi elektrolit memiliki pengaruh yang signifikan dalam pembentukan susunan nanotube. Menurut perbedaan elektrolit yang kami gunakan, pengembangan pada dasarnya dibagi menjadi tiga tahap:Tabel 1 merangkum kondisi anodisasi dan dimensi TiO yang dihasilkan₂ susunan nanotube dalam tiga generasi yang diselidiki oleh berbagai kelompok penelitian hingga saat ini.

Generasi pertama:elektrolit berair berbasis asam fluorida (HF)

Tonggaknya adalah bahwa Gong et al. untuk pertama kalinya menyajikan susunan tabung nano titania yang seragam dengan oksidasi anodik Ti dalam elektrolit berair berbasis HF [54]. Dalam larutan elektrolit HF berair, di mana pH relatif rendah yang berarti konsentrasi ion hidrogen tinggi, pelarutan kimia TiO₂ diinduksi oleh ion fluor memainkan status dominan dalam proses anodisasi [55]. Kesetimbangan dinamis dicapai dalam waktu singkat dalam proses pembentukan nanotube titania, dan oleh karena itu, panjang nanotube maksimum yang dapat dicapai dibatasi hingga sekitar 0,5 m [54,55,56].

Generasi kedua:elektrolit buffer

Dalam pekerjaan selanjutnya, untuk mengurangi pembubaran kimia memperpanjang tabung, Cai et al. menunjukkan bahwa dengan menambahkan asam lemah seperti KF atau NaF ke dalam larutan buffer dan menyesuaikan pH menjadi asam lemah (pH =4,5) dengan asam sulfat atau natrium hidroksida, nanotube sekitar 4,4 m panjangnya tercapai [57]. Nilai PH mempengaruhi hidrolisis ion titanium yang ternyata mengganggu etsa elektrokimia dan pelarutan kimia. Cai dkk. juga menunjukkan bahwa nilai pH yang lebih rendah menghasilkan nanotube yang lebih pendek tetapi bersih dan nilai pH yang lebih tinggi menghasilkan nanotube yang lebih panjang tetapi puing-puing yang tidak diinginkan [57]. Ketika nilai pH naik, laju hidrolisis akan meningkat, pada gilirannya memperlambat pelarutan kimia, yang mengarah ke nanotube lebih lama sementara larutan basa tidak cocok untuk pertumbuhan nanotube [57, 58]. Hal ini menunjukkan bahwa dalam elektrolit NaF netral pada tegangan yang tepat nanotube lebih lama dapat diperoleh daripada dalam larutan asam oleh Macak et al. [58]. Mengingat tegangan tertentu dalam elektrolit yang mengandung fluorida, dengan menyesuaikan gradien pH, rasio aspek yang diperlukan dan ketebalan lapisan dapat dicapai [59].

Generasi ketiga:elektrolit organik polar

Elektrolit seperti gliserol [59], dimetil sulfoksida [60], formamida atau dietilen glikol [61, 62], etilen glikol [41, 63], yang mengandung spesies fluorida seperti NH₄ F, NaF, dan KF secara bertahap muncul. Macak dan rekan kerja memimpin dalam menggunakan elektrolit gliserol kental untuk membuat array titania nanotube dengan ketebalan sekitar 7 m dan diameter tabung rata-rata 40 nm [59]. Hal ini menunjukkan bahwa rasio aspek yang lebih tinggi TiO₂ nanotube dapat tumbuh dalam elektrolit organik polar seperti karena kontrol yang tepat dari pH elektrolit mengurangi pembubaran kimia titania [64]. Paulose dkk. membentuk nanotube dengan panjang kira-kira 134 m, dibuat menggunakan etilen glikol yang mengandung 0,25 wt% NH₄ F pada potensial anodisasi 60 V selama 17 jam [60]. Segera setelah itu, TiO setebal lebih dari 250 m₂ array nanotube dilaporkan oleh Albu [65]. Selain itu, kandungan air memainkan peran ganda dalam proses:sangat diperlukan untuk pembentukan titania, tetapi juga mempercepat pembubaran kimia [63]. Oleh karena itu, cara mengecilkan pengaruh kadar air menjadi signifikansi minimal diperlukan untuk meningkatkan ketebalan dan derajat orde TiO₂ array nanotube. Secara umum, membatasi kadar air hingga kurang dari 5% adalah kunci untuk mencapai tabung nano yang sangat panjang dengan sukses [60], dan jumlah minimum kadar air (0,18% berat) diperlukan untuk membentuk tabung nano titania yang terorganisir dengan baik [66]. Dilaporkan bahwa dengan penambahan air, kerapatan arus tercatat menurun yang tertinggi dalam larutan etilen glikol anhidrat [66]. Paulose dkk. pertama kali melaporkan pembentukan susunan heksagonal titania nanotube dengan panjang sekitar 1000 m pada 60 V selama 216 jam dalam etilen glikol yang mengandung 0,6 berat NH₄ F dan 3,5% air [41]. Fenomena lain yang terlihat adalah bahwa dinding tabung halus tumbuh pada kadar air rendah, sementara riak pada dinding samping terbentuk pada kadar yang lebih tinggi seperti yang ditunjukkan pada Gambar. 4b [59, 67]. Sejauh ini jenis elektrolit yang paling banyak digunakan, etilen glikol yang mengandung air dan ion fluorida selalu mengarah ke struktur nanotube berdinding ganda (Gbr. 4d) [40, 68,69,70], sedangkan lapisan dalam dapat dihilangkan dengan perlakuan anil yang sesuai diikuti dengan proses etsa kimia sederhana. Setelah pelepasan cangkang bagian dalam, tabung yang melebar memungkinkan dekorasi lapis demi lapis dengan nanopartikel menggunakan pendekatan berulang berdasarkan TiCl₄ -hidrolisis [71]. Sedangkan tabung berdinding tunggal menunjukkan konduktivitas yang ditingkatkan secara signifikan dan waktu transpor elektron dalam sel surya peka pewarna (DSSCs) [71, 72] di mana ketebalan seluruh tabung pada dasarnya sama dan kulit bagian dalam tidak lagi muncul, Mirabolghasemi et al. membuat perbandingan antara tabung berdinding ganda dan tunggal dan disajikan tabung berdinding tunggal yang diinginkan dengan penambahan dimetil sulfoksida (DMSO) ke dalam elektrolit dengan 1,5 M H₂ O dan 0,1 M NH₄ F [72].

Baru-baru ini, elektrolit berbasis non-fluoride telah dilaporkan menumbuhkan TiO₂ array nanotube yang dapat dianggap sebagai generasi sintesis keempat termasuk asam klorida, hidrogen peroksida, larutan asam perklorat, dan campurannya [73, 74]. Allama dan Grimes menjelaskan susunan nanotube yang dikembangkan dengan baik dengan panjang 300 nm, diameter dalam 15 nm, dan diameter luar 25 nm diperoleh dalam elektrolit berair asam klorida (HCl) 3 M pada tegangan oksidasi antara 10 dan 13 V. Tetapi menambahkan konsentrasi rendah H₃ PO₄ mengakibatkan perubahan dari nanotube ke batang. Mereka lebih lanjut menyarankan bahwa mereka tidak dapat mencapai susunan nanotube yang terorganisir sendiri dalam elektrolit yang mengandung HCl pada konsentrasi lebih rendah atau lebih tinggi dari 3 M [73]. Allama menemukan bahwa menambahkan hidrogen peroksida ke asam klorida yang mengandung larutan berair dapat menjadi metode yang memungkinkan untuk memperpanjang tabung nano titania yang memiliki sifat pengoksidasi kuat mengikuti lapisan oksida yang lebih tebal, menunjukkan bahwa ion fluorida dapat berhasil digantikan oleh ion klorida dalam pertumbuhan. dari array nanotube [74]. Selain itu, cairan ionik tanpa penambahan spesies fluorida bebas telah diperlakukan sebagai jenis lain dari sistem pelarut untuk titania nanotube dalam beberapa tahun terakhir [75, 76].

Selain parameter standar, geometri nanotube yang dihasilkan bergantung pada penggunaan elektrolit yang berulang (“efek larutan bekas”). Dibandingkan dengan tabung yang diperoleh dengan larutan segar, menggunakan larutan sekali pakai, menunjukkan peningkatan panjang tabung nano dan kualitas yang lebih baik di mana laju pertumbuhan tabung nano yang dicapai secara konsisten lebih tinggi untuk larutan sekali pakai pada 60 V ke atas [77] dan perilaku transien arus yang sedikit berbeda tetapi dapat dibedakan dapat dicatat [66]. Selain itu, tidak ada struktur nanotubular tetapi film oksida diperoleh dalam larutan yang digunakan dua kali karena penipisan F ⁻ spesies [78]. Namun, Sopha et al. menyelidiki perbedaan umur elektrolit berbasis etilen glikol pada morfologi TiO₂ nanotube menunjukkan bahwa dalam elektrolit yang lebih tua susunannya menunjukkan rasio aspek yang lebih rendah [79].

Potensi Terapan

Tegangan anodisasi adalah faktor kritis yang mengendalikan diameter tabung [80, 81]. Dimensi dari susunan nanotube dapat diprediksi hanya dengan menerapkan kisaran tegangan yang sesuai yang disebut jendela potensial melintasi elektroda [67]. Pada tegangan rendah, pelepasan medan listrik lebih sedikit, membentuk TiO₂ nanotube dengan diameter lebih kecil. Jika tegangan terlalu rendah, TiO₂ lapisan menjadi kompak tetapi tidak ada struktur nanotubular yang dapat diamati. Sebaliknya, struktur berpori seperti spons akan terlihat ketika tegangan terlalu tinggi. Dengan tegangan yang dapat dikontrol, diameter nanotube sebanding dengan tegangan [81]. Lebih lanjut, penelitian menunjukkan bahwa kisaran tegangan pembentuk nanotube juga terkait dengan sistem elektrolit. In aqueous electrolytes, the potential window should be controlled from 10 to 25 V, which in organic electrolytes is much wider between several volts and some hundred volts. Wang and Lin found out the fact that in aqueous electrolytes, the anodization potential exhibits significant influence on the growth of TiO₂ nanotube arrays, which exhibited slight influence in non-aqueous electrolytes in this regard [82]. The voltage dependence has a significant reduction in non-aqueous electrolytes which is attributed to a large extent to the low conductivity of organic electrolytes [83, 84].

The Duration on Anodization

The duration of anodization affects the nanotubes mainly in two aspects:(I) the formation of the tubes or not and (II) the length of the tubes. That is, in the early stage of the anodization, a compact TiO₂ film is formed. If the duration is too short to reach an equilibrium in reaction, the regular nanotube array cannot be achieved instead of a disordered porous layer [67]. With increasing the anodization time, porous structure gradually grows deeper and converts into the TiO₂ nanotubular array [1, 33, 51]. If other electrochemical parameters are kept unchanged, increase in the nanotube length is observed over time while no significant effect on diameter and tube wall thickness until a steady-state situation occurs [67, 85, 86]. However, due to the decrease of the F ⁻ concentration in the electrolyte, where the ion transport rate decreased, the growth rate of nanotubes is reduced. After reaching a stable condition between tube growth at the bottom and chemical/electrochemical dissolution at the top, we will find no further increase in length of the nanotubes [87]. As time continues to go, pipe orifice becomes an irregular polygon resulting in TiO₂ spikes and coverings which can be seen on the surface of the TiO₂ nanotube arrays [36]. It is worth mentioning that enlightened by the success of aluminum-repeated anodization for self-organized porous alumina [88], the two-step anodization of titanium for such a highly ordered hexagonally packed nanostructure of titania has appeared [43, 77, 89,90,91]. After the first-step anodization, the first nanotube layer from the Ti foil should be removed ultrasonically or by using an adhesion tape which leads to a surface where the remaining Ti is covered by comparably ordered dimples. Researches have shown that the former treatment helps to avoid potential mechanical damage to the Ti surface and also improve the structural uniformity of the TiO₂ nanotubes to a great extent [77, 90, 91]. In the second anodization step, the pretreated Ti foil would be used as anode again with or without changes in parameters of oxidation conditions. It is subsequently found that the highly ordered and vertically oriented titania nanotubes, have greater potential in such fields as photocatalysis [77], photoelectriochemical activity [92, 93], and biological interaction with cells [94] than the disordered nanotubular titania.

Electrolyte Temperature

Temperature restricts the growth and quality of titania nanotube arrays, directly affecting the rate of oxide growth, length, and wall thickness of the structure [64, 95]. Wang and Lin first reported the effect of electrolyte temperature in both aqueous and non-aqueous electrolyte on anodic oxidation of titanium [82]. In aqueous electrolyte, with the temperature increasing, a slight diminish in the internal diameters was observed while the external diameters remained the same [68]. The reason may be the dissolution induced by electrical field and fluoride ions are similar while the oxide formation rate is higher than that at lower temperature. In non-aqueous electrolyte containing fluoride ions, the outer nanotube diameter was found to be largely increased by the increasing electrolyte temperature [82]. This may be because at lower temperature, the ion mobility of fluorine in some viscous electrolyte is further inhibited, leading to much slower dissolution of newly formed titania, which subsequently lead to a smaller nanotube diameter. As chemical dissolution rate increases, surface of TiO₂ nanotubes arrays can easily produce excessive corrosion, resulting in lodging nanotubes and agglomeration. Therefore, the appropriate bath temperature for stable TiO₂ nanotube arrays is at room temperature [82, 95, 96].

Modification of Nanotubes Properties

Increasing applications of TiO₂ nanotubes as a novel semiconductor are closely related to its photoelectriochemical (PEC) performance; however, they are sometimes prevented by two fundamental drawbacks:(I) the wide band gap (3.0 eV for the rutile phase and 3.2 eV for the anatase phase) can only absorb ultraviolet light, which accounts for less than 10% of the sunlight [97], resulting in low average utilization ratio of solar energy and (II) the low electrical conductivity cannot efficiently transfer photogenerated carries. At the same time, the photoelectrons and vacancies can be easily recombined, thus making low electron mobility rate or quantum confinement effects [98]. Hence, post-treatment of TiO₂ nanotubes is the key to improve the performance of its materials and related devices successfully. Considerable researches have been reported on modified methods to reduce the recombination of photogenerated electron-hole pair rate, speed up the electron transfer rate, and enhance the photoelectriochemical activity of TiO₂ nanotube. The research of the methods for the improvement of the photoelectriochemical properties of TiO₂ nanotubes will be reviewed, including thermal annealing, doping, and surface modification. As for promising modification in biomedical fields, we will present in the application section.

Thermal Annealing

The crystallinity of the nanotube arrays and their conductivity, lifetime of charge carrier, and photoresponse depend mainly on the thermal annealing temperature and atmosphere [99, 100]. The as-prepared TiO₂ nanotubes above are amorphous in nature but can be annealed to anatase or rutile phase, or mixtures of both phases relying on the specific temperature [1, 3, 40, 92, 100]. It is demonstrated that amorphous nanotube layers grown in a glycerol-based electrolyte containing fluoride ions have low photocurrents and an incident photon-to-electron conversion efficiency (IPCE) below 5% due to lots of structural defects while anatase phase nanotubes exhibit an IPCE value up to 60% thus attracting more interest to applications such as dye-sensitized or perovskite solar cells [93]. As well in mixed water-glycerol electrolyte with F ⁻ , Das et al. stated their points that if the self-organized TiO₂ nanotube arrays with thickness about 1 μm were annealed around 300–500 °C, the anatase phase of TiO₂ as the most preferred crystalline structure could be observed. The single anatase structure of nanotubes with the best photoelectriochemical properties and the lowest resistivity could be fabricated when annealed at 400 °C. At temperature higher than 600 °C, a track of typical rutile appeared and with a further increase in annealing temperature the percentage and quality of the rutile phase increased [92]. It should be noted that in Jaroenworaluc’s work, rutile phase was detected in anodic nanotube layers grown in aqueous NaF/Na₂ JADI₄ with thickness of approximately 1.5 μm at 500 °C heat treatment and became the dominant phase at 600 °C. Whereas at 550 °C, partial nanotubes began to break down [101]. It begins to cause the collapse of the entire nanostructure formed in aqueous NaF/Na₂ JADI₄ with the continuous increase of temperature (800–900 °C) or the extended annealing time [3]. While for extended temperature, the crystalline structure of the nanotubes completely converts to rutile phase at above 900 °C [3]. Some researchers demonstrated a loss of the typical single-walled nanotube layers morphology when the annealed temperature rose above 580 °C [102]. Besides the whole annealing process especially the heating rate controls, the morphological structures of the entire nanotube arrays [40]. The double-walled nanotube layers prepared from ethylene glycol (containing less than 0.2 wt% H₂ O), with the addition of HF and H₂ O₂ , have such a high stability that can keep their structure intact until temperature is higher than 900 °C with a heating rate of 1 °C s ⁻¹ . However, the double-walled nanotubes begin to collapse as soon as the temperature reaches 500 °C when the heating rate is 25 °C s ⁻¹ . Most extraordinarily, with the high speed of 50 °C s ⁻¹ the entire separated nanotubes fuse into a highly ordered porous membrane [40]. Xiao et al. obtained crystallized titania nanotubes arrays with calcination in different gases like dry nitrogen, air, and argon indicating nanotubes in dry nitrogen appeared to have enhanced electrochemical and photoelectrical properties who also found out that with the increasing temperature internal diameter decreased while wall thickness increased at the expense of nanotubes length [103].

As shown in Fig. 8, the conductivity along the TiO₂ nanotubes with three different thickness is strongly affected by annealing temperature. Smallest resistance is observed at about 350–450 °C when the amorphous nanotube arrays are totally converted into anatase layers [99]. And it is evident to see that specific resistivity increases with thicker nanotube arrays which can be shown more clearly in the inset in Fig. 8. Furthermore, calcination temperature is responsible for the decrease in the length of the anatase TiO₂ nanotube. As shown in Fig. 9a, increment of temperature between 300 and 500 °C causes the as-prepared nanotube arrays slightly changing in thickness from 13.6 to 12.6 μm. When annealing temperature continuously increases to 600 °C, the average length of the nanotubes decrease dramatically to 6.6 μm. Figure 9b shows conversion from anatase TiO₂ to rutile phase TiO₂ occuring at 500 °C when the rutile barrier layer is formed on the bottom of the TiO₂ nanotube arrays along the anatase nanotubes by consuming the bottom layer if the annealed temperature is further increased. This leads to a length decrease and corresponding photocatalytic activity decline [104].

Electrical resistance as a function of the annealing temperature for the different nanotube layer thicknesses. The curve shows electrical resistance measurement for different titania nanotube arrays grown in ethylene glycol based electrolyte containing HF and water at different temperature and the influence of thickness on resistance. The inset shows more details about the relationship between the thickness of the nanotube arrays annealed at 250 °C and their specific resistivity. Reproduced from ref. [99]

Evolution of titania nanotube arrays at different calcination temperatures. The electrolyte was ethylene glycol containing 0.3 wt% ammonium fluoride and 5 vol% distilled water. a The decrease in the thickness of titania nanotube arrays at different annealing temperature from 300 to 600 °C. The insets are corresponding SEM images and the scale bar is 5 μm. b The schematic of crystallization process of anodic titania nanotubes annealed at (1) 450 °C, (2) 500 °C, and (3) 600 °C in air. Reproduced from ref. [104]

Doping

Doping ions or atoms into titania lattice, a substitution within the lattice either at Ti ⁴⁺ or O ²⁻ sites, on the one hand, changes the lattice constants and bond energy. On the other hand, it is beneficial to the separation between photogenerated electron and hole pair, which in turn adjusts the band gap and improves the photoelectrochemical performance of nanotubes [15]. The impurity doping has been commonly applied to extend the light absorption onset of TiO₂ nanotubes by either introducing subbandgap states or adjusting its bandgap width [105]. Lately, co-doping approach has been proposed as a more efficient way to reduce the band gap and adjust energy band level in favor of photoelectriochemical reactions [106, 107]. There are various kinds of doped-elements and preparation methods, and Table 2 summarizes some methods and the doping effects of doped titania nanotubes.

The most typical doped TiO₂ nanotubes are as follows:

1. i.

   Metal-doped TiO₂ nanotubes such as Nb [107], Fe [108], Cu [109], Cr [110], Zr [111], Zn [112], and V [113]

2. ii.

   Non-metal-doped TiO₂ nanotubes such as N [105], F [114], B [115], C [116], S [117], and I [118]

3. iii.

   Co-doped TiO₂ nanotubes such as N–Ta [105], N–Nb [107], and C–N–Ni [119]

Choiet systematically studied the photoreactivities of 21 metal ion-doped quantum-sized TiO₂ doping with Fe, Mo, Ru, Os, Re, V, and Rh significantly increases quantum efficiency, while Co and Al doping decreases the photoreactivity [120]. Momeni et al. recently obtained Fe-TiO₂ nanotube (Fe-TNT) composites using different amounts of irons to decorate anodically formed TiO₂ nanotubes with potassium ferricyanide as the iron source, indicating that Fe doping efficiently accelerates the photocatalytic performance for water splitting [108]. Not limited to transition metals, other elements including N [105], F [114], B [115], C [116], S [117], and I [118] are successfully explored. Nitrogen-doped TiO₂ nanotubes turns out to be a promising path to narrow the band gap energy with enhanced photocurrent response in the visible light and the tube length influences the magnitude of conversion efficiency [121, 122]. Kim and co-workers proved that TaOxNy layer-decorated N-TNT (N-doped TiO₂ nanotubes) as dual modified TNTs have significantly improved both visible (3.6 times) and UV (1.8 times) activities for water splitting [105]. At present, more researches are aimed at co-doping which exhibits remarkable synergistic effect causing a significant improvement on photoelectriochemical properties. Chai et al. grew Gd–La co-doped TiO₂ nanotubes by an ultrasonic hydrothermal method, enhancing visible light photocatalysts [123]. Cottineau et al. modified titania nanotubes with nitrogen and niobium to achieve co-doped nanotubes with noticeably enhanced photoelectriochemical conversion efficiency in the visible light range [107]. Nevertheless, the mechanism for increasing photoconductivity and synergistic effect of various elements on co-doping remains a further study.

Surface Modification

Surface modification means decoration on surface of TiO₂ nanotube arrays with nanoparticles (metal, semiconductors, and organic dyes). Nanowire arrays can also be fabricated by electrodeposition into titanium oxide nanotubes [124]. TiO₂ nanotube is a semiconductor with a wide band gap, which can only absorb ultraviolet light [97, 125]. Any other nanomaterials which possess a narrow band gap or can absorb the visible light can be used as a sensitizer for titania nanotubes. Silver nanoparticles can be decorated on the tube wall by soaking the titania nanotube arrays in AgNO₃ solutions and photocatalytically reducing Ag ⁺ on a TiO₂ surface by UV illumination [126]. Ag/TiO₂ nanotubes show a significantly higher photocatalytic activity and good biological performance compared with neat TiO₂ nanotubes [126, 127]. Some compositions such as graphene oxide GO [128], CdS [129], CdSe [130], and ZnFe₂ O₄ [131]. can be modified on TiO₂ nanotube arrays. Lately, GO have attracted much scientific interest in nanoscale devices and sensors which is easy to combine with nanostructure materials to compose some compounds. Titania nanotubes fabricated by anodization in water-ethylene glycol electrolyte consisting of 0.5 wt% ammonium fluoride (NH₄ F) can be incorporated with GO by cyclic voltammetric method, which achieve higher photocatalytic activity and more effective conversion efficiency (GO-modified vs pure nanotubes =26.55%:7.3%) of solar cell than unmodified TiO₂ nanotubes [128]. Semiconductor composite is a method improving the performance of titania nanotubes via, in some specific way, combining two kinds of semiconductors with different band gap [132]. Yang dkk. decorated CdSe nanoparticles on the surface of TiO₂ nanotubes by applying an external electric field to accelerate CdSe nanoparticles in nanochannels resulting in a material with more stable and higher photoresponse to visible light. Furthermore, the degeneration rate of anthracene-9-carbonxylic acid when exposed to the green light irradiation indicating that CdSe dominates the photocatalytic process under visible light [130].

Besides, other oxide nanoparticle deposition such as WO₃ [133] or TiO₂ [134] onto TiO₂ nanotubes by the hydrolysis of a chloride precursor also turns out to augment the surface area and improve the solar cell efficiency. Another very effective approach is to consider organic dyes as sensitizers for TiO₂ nanotubes to improve its optical properties [135]. Lately, atomic layer deposition (ALD) becomes an established procedure to modify TiO₂ nanotube layers. ALD appears to be a very uniform and precisely controllable deposition process to functionalize nanotubes in conformably coating the surface of the nanotube layers with one atomic layer after another of a secondary material, such as Pd [136], ZnO [137], Al₂ O₃ [138], CdS [139], or TiO₂ [140].

Biomedical Applications

Historically, the mentioned milestones were reported on the fabrication of titania nanotube arrays contributing to widen the promising applications over the past 20 years in the areas ranging from anticorrosion, self-cleaning coatings, and paints to sensors [141,142,143], dye-sensitized and solid-state bulk heterojunction solar cells [144,145,146], photocatalysis [147, 148], eletrocatalysis, and water photoelectrolysis [149, 150]. They also outperform in biomedical directions as biocompatible materials, toward biomedical coatings with enhanced osseointegration, drug delivery systems, and advanced tissue engineering [15, 135, 141, 142, 151]. In the following section, we will give an overview of current efforts toward TiO₂ nanotubes biomedical applications. Titania nanotubes possess good biocompatibility as they show some antibacterial property, low cytotoxicity, good stability, and cytocompatibility including promoting adhesion, proliferation, and differentiation of osteoblast and mesenchymal stem cells (MSCs) with a high surface area-to-volume ratio and controllable dimensions [152,153,154,155].

However, Ti products have inadequate antibacterial ability and efforts have been made to improve their antibacterial properties such as modifications on titania nanotubes for biomedical applications like bioimplant [126, 156].

Biological Coatings And Interactions with Cells

A number of in vitro and in vivo studies have demonstrated that MSCs, osteoblasts and osteoclasts show size-selective response which means the effect of size holds an important position in cell interaction where the optimized size for cell adhesion, proliferation, growth, and differentiation is ranging from 15 to 100 nm [153, 157, 158]. Particularly, it was demonstrated that the TiO₂ nanotubes with a diameter of 70 nm was the optimal nanoscale geometry for the osteogenic differentiation of human adipose-derived stem cells (hASCs) [159]. Smith et al. reported increased dermal fibroblasts and decreased epidermal keratinocyte adhesion, proliferation, and differentiation on TiO₂ nanotube arrays (diameter 70–90 nm, length 1–1.5 μm) [160]. As shown in Fig. 10, Peng et al. found that nanotubular surface preferentially promoted proliferation and function in endothelial cells (EC) while decreased in vascular smooth muscle cell (VSMC) by measuring EdU, a thymidine analog which is incorporated by proliferating cells [161]. Furthermore, it is pointed out that surface wettability of the TiO₂ nanotube layers is recognized as a critical factor for cell behavior which can be adjusted by changing the diameter of the nanotubes. That is to say, water contact angles can be altered without changing the surface chemistry [158]. To get further understanding of the effect of TiO₂ nanotube layers to bone-forming cells as well as stem cells response, Park et al. seeded green fluorescent protein-labeled rat MSCs on TiO₂ nanotube layers with six different diameters (15, 20, 30, 50, 70, and 100 nm), resulting in cell activity that is sensitive to nanoscale surface topography with a maximum in cell activity obtained for tube diameters of approximately 15–30 nm. Such lateral spacing exactly corresponds to the predicted lateral spacing of integrin receptors in focal contacts on the extracellular matrix, forcing clustering of integrins into the closest packing, resulting in optimal integrin activation. While tube diameters larger than 50 nm, severely impaired cell spreading, adhesion, and spacing of 100 nm may lead to the cell apoptosis [94]. Besides adjusting the size of the nanotubes, surface modification loaded with bioactive factors should be highlighted, in which case biomedical properties can be further optimized. In the case of bone implants, hydroxyapatite (HA) formation is important for osseointegration. Recent works have shown hydroxyapatite nanocrystalline coating onto the nanotubular TiO₂ results in further enhanced osseointegration with strong adhesion and bond strength, and a drastic enhancement of deposition rate is observed [162, 163]. Nanotubular TiO₂ surface can greatly enhance the natural apatite growth rate in simulated body fluid (SBF) compared with flat surfaces [10, 164]. The alkaline-treated TiO₂ nanotubes with NaOH solutions are more bioactive in SBF, where sodium titanate can significantly accelerate nucleation and the growth of HA formation presenting a well-adhered bioactive surface layer on Ti due to its larger surface area and promoted mechanical interlocking between HA and TiO₂ nanotubes [165, 166]. Electrodeposited with hydroxyapatite, higher adhesion of TiO₂ nanotubes has been described in the literature by means of adhesive tape test and the live/dead cell staining study which is essential for early bone formation [166]. The results also showed that at the length of 560 nm the highest adhesion of HA surface on the nanotubes is observed. Also the nanotube surface can indeed strengthen Collagen type I expression in vivo experiment which is considered to be a basic initial bone matrix protein in bone formation [167]. Moreover, annealing of the amorphous nanotubes to anatase or a mixture of anatase and rutile was found to be an important factor in the apatite formation process [164].

Ratio of EdU positive a ECs and b VSMCs on flat or nanotube substrate. It is normalized by the average proportion of positive cells on flat surfaces on day 1 and 3. Data is presented as average ± standard deviation. *p < 0.05, **p < 0.01 versus same day flat control, n = 6 reproduced from ref. [161]

Drug Delivery and Antibacterial Ability

Furthermore, the tubular nature of TiO₂ in biomedical devices may be exploited as gene and drug delivery carriers with living matter due to its high surface area, controllable pore, and self-ordered structure [1, 15]. When the orthopedic bioimplant is placed into the bone defect, persistent and chronic infection is one of the most common and serious complications associated with biomedical implantation [16, 168]. Certain dimension and crystallinity may be useful to prevent bacteria adhesion and promote bone formation. The thermal annealing has decreased the number of bacteria adhering to the Ti surface. It could be in part because heat treatment removes the fluorine content which has a tendency to attract bacteria. The research also indicates that nanotubes with 60 or 80 nm in diameter decrease the number of live bacteria as compared to lower diameter (20 or 40 nm) nanotubes [169, 170].

Bauer et al. loaded epidermal growth factor (EGF) and bone morphogenetic protein-2(BMP-2) onto the TiO₂ nanotubes surface by covalent attachment. They observed positive influence on the behavior of MSCs on 100-nm nanotube arrays where cell count was at much higher levels compared to the untreated one [171]. Lately, titania nanotubes loaded with antibiotics contribute to suppressing bacterial infections. As gentamicin sulphate (GS) is mostly widely used with highly water solubility, Feng et al. loaded titania nanotubes with GS through physical adsorption and cyclic loading which can treat many types of bacterial infections [172]. Zhang dkk. fabricated titania nanotubes loaded with vancomycin to investigate the increasing biocompatibility and obvious antibacterial effect on Staphylococcus aureus [173]. However, systemic antibiotics in clinical will bring many side effects. The release of antibiotics from the nanotubes is too fast to maintain the long-term antibacterial ability, and the use of antibiotics may develop resistant strains [126, 168, 174]. Ensuring a constant release rate becomes a crucial but difficult part in the field of drug delivery. In strategies like surface modification, controlling the dimension of nanotube arrays, biodegradable polymer coating have been employed to solve the issue [21]. Drug release of several drugs such as antibiotics or growth factors from titania nanotube arrays can be adjusted by varying their diameters and lengths [152, 175, 176]. Feng dkk. covered a thin film comprising a mixture of GS and chitosan on GS-loaded titania nanotubes and showed a controlled release of the drug providing sustained release effects to a certain extent [172]. Titania nanotube arrays as drug nanoreservoirs on Ti surface for loading of BMP-2 were fabricated by Hu et al. and then further covered with gelatin/chitosan multilayers to control the release of the functional molecule meanwhile maintain the bioactivity for over 120 h via a spin-assisted layer-by-layer assembly technique which is mainly based on electrostatic interactions between polyanions and polycations as well as promote osteoblastic differentiation of MSCs [177]. Lai et al. successfully fabricated Chi/Gel multilayer on melatonin-loaded TiO₂ nanotube arrays to control the sustained release of melatonin and promote the osteogenic differentiation of mesenchymal stem cells [178]. Karan et al. synthesized titania nanotubes loaded with the water-insoluble anti-inflammatory drug indomethacin and modified lactic-co-glycolic acid on surface as a polymer film in order to extend the drug release time of titania nanotubes and produce favorable bone cell adhesion properties, with reduced burst release (from 77 to> 20%) and extended overall release from 4 days to more than 30 days [152]. As previous study reported that surface treatment of implants with N -acetyl cysteine (NAC) may reduce implant-induced inflammation and promote faster bone regeneration [179], Lee et al. examined the feasibility of N -acetyl cysteine-loaded titania nanotubes as a potential drug delivery system onto an implant surface, and the data indicates the enhanced osseointegration and the value of the small animal model in assessing diverse biological responses to dental implants. Besides, TiO₂ nanotube arrays are suitable for loading inorganic agents like Ag, Sr, and Zn to obtain long-term antibacterial ability and osseointegration [126, 180,181,182]. Ag nanoparticles have been incorporated into TiO₂ nanotube arrays previously with satisfactory small possibility to develop resistant strains, a broad-spectrum antibacterial property, low cytotoxicity, and good stability by immersion in a silver nitrate solution followed by ultraviolet light radiation [126]. Zhang dkk. demonstrated that a series of porous TiO₂ coatings with different concentrations of silver had significant inhibition effect on Escherichia coli and Staphylococcus aureus . Besides, only with the optimum amount of silver can the coatings retain the antibacterial effect but without any measurable cytotoxicity to cells [183]. Due to cytotoxicity observed by the excessive release of Ag ⁺ subsequently, titania nanotube arrays with Ag₂ O nanoparticles embedded in the wall are prepared on Ti by TiAg magnetron sputtering and anodization in order to get slower and more controllable silver ion release [184]. That is because the TiO₂ barrier is surrounded thereby minimizing the cytotoxicity induced by burst or large Ag ⁺ release.

Similar to Ag, Zn possesses antibacterial and anti-inflammation properties, and osteogenesis induction [185,186,187]. Huo et al. produced anodic TiO₂ nanotube arrays at 10 V and 40 V (NT10 and NT40) incorporated with Zn by hydrothermal treatment at 200 °C for 1 and 3 h (NT10-Zn1, NT10-Zn3, NT40-Zn1, and NT40-Zn3) in Zn containing solutions, followed by annealing at 450 °C for 3 h in air. NT40-Zn3 has the largest Zn loading capacity and releases more Zn compared with other samples. The amounts of Zn released diminish gradually with time and nearly no Zn can be detected 1 month later except sample NT40-Zn3 (Fig. 11). The NT-Zn samples present different antibacterial ability. It is evident that NT40-Zn3 and NT10-Zn3 effectively kill more adherent bacteria as well as surrounding planktonic bacteria in the early stage. Figure 12a describes a synergistic effect of both released and surface incorporated Zn while Fig. 12b explains the effect of the released Zn [181].

a Total amounts of Zn incorporated into the NT-Zn samples for the 1 cm ² coatings and b non-cumulative Zn release profiles from NT-Zn into PBS. Reproduced from ref. [181]

a Antibacterial rates versus adherent bacteria on the specimen (Ra) and b antibacterial rates against planktonic bacteria in the medium (Rp) *, **p  < 0.05 and 0.01 vs NT10; ^# , ^## p  < 0.05 and 0.01 vs NT40; ^★ , ^★★ p  < 0.05 and 0.01 vs NT10-Zn1; ^% , ^%% p  < 0.05 and 0.01 vs NT10-Zn3; ^$ , ^$$ p  < 0.05 and 0.01 vs NT40-Zn1. Reproduced from ref. [181]

Kesimpulan

This review presents the historical developments and traditional formation mechanism of titania nanotube arrays grown by electrochemical anodization as well as the approaches to influence and modify morphology in order to improve their performances. We also focus on current efforts toward TiO₂ nanotubes applications in biomedical directions. Those steady progresses have demonstrated that TiO₂ nanotubes are playing and will continue to play an important role in material science, but there are still some aspects needed to be further improved.

1. 1.

   The synthesis of TiO₂ nanotube arrays is already comparatively mature so far in fact, but how to simplify the technology for the purpose of large-scale production in industry with extending practical operability and how to precisely control nanotube geometry efficiently by varying the anodic parameters so as to obtain optimized properties have yet to be further investigated.

2. 2.

   The formation mechanisms of anodic TiO₂ nanotubes have gradually become a hotspot of research due to their unique structure and excellent performances but the exact mechanism remains controversial. Conventional FAD explains the growth process and the porous structure of TiO₂ nanotubes, but the combination of viscous flow model and growth model of two currents can give a comprehensive explanation to the growth process. Notably, the validity of oxygen evolution resulting from electronic current has much room for investigation.

3. 3.

   Modification is key for improving performances of titania nanotube arrays. Thus, we need to explore more methods for modification and take full advantage of the self-organized nanostructure. Through self-assembling inorganic, organic, metallic, and magnetic nanoparticles into or onto the tubes as nanocomposites with broad spectral response to visible light, high quantum efficiency, and stabilizing properties, applications could be widened. Currently, ALD appears to be an option to coat the titania nanotube layers homogenously and precisely from the bottom to the tube mouth, resulting in many advanced functionalities of the newly prepared nanotube layers. Nevertheless, further optimization of the ALD process toward coatings and inner fillings is demanded.

4. 4.

   TiO₂ nanotube researches in biomedical directions are still in their infancy and have a long distance to go in clinical use. The biological reaction between cells and titania nanotubes has to develop from cellular level to molecular level and from morphological changes to molecular alterations. It has been shown that adhesion, spreading, and growth of osteoblast and mesenchymal stem cells strongly depends on nanotube diameter, so the regularity and principle of this phenomenon as well as other factors affecting cells’ behaviors need to be further explored.

Singkatan

ALD:

Deposisi lapisan atom

BMP-2:

Bone morphogenetic protein-2

DMSO:

Dimetil sulfoksida

DSSCs:

Dye-sensitized solar cells

EBSD:

Electron Backscatter Diffraction

EC:

Endothelial cells

EdU:

A thymidine analog

EGF:

Epidermal growth factor

FAD:

Conventional field-assisted dissolution

FE-SEM:

Mikroskop elektron pemindaian emisi medan

Fe-TNTs:

Fe-doped TiO2 nanotubes

FIB:

Focused ion beam

GO:

Grafena oksida

GS:

Gentamicin sulphate

HA:

Hydroxyapatite

hASCs:

Adipose-derived stem cells

IPCE:

Incident photon-to-electron conversion efficiency

MSCs:

Mesenchymal stem cells

NAC:

T -Acetyl cysteine

N-TNT:

N-doped TiO2 nanotubes

PEC:

Photoelectriochemical

SBF:

Simulated body fluid

UV:

Ultraviolet

VSMC:

Vascular smooth muscle cell

Peningkatan efisiensi konversi fotolistrik untuk sel surya peka pewarna berserat fleksibel Millstone Exfoliation:True Shear Exfoliation untuk Graphene Oxide Berukuran Besar Sedikit