Titik Karbon sebagai Material Generasi Baru untuk Nanotermometer:Tinjau

Titik Karbon sebagai Nanotermometer

Untuk mengatasi masalah yang muncul dari nanotermometer non-karbon (seperti yang kami jelaskan di bagian sebelumnya), nanomaterial berbasis karbon telah disiapkan dan menampilkan sifat unik, seperti toksisitas rendah, persiapan mudah, prekursor murah, fotostabilitas. , dan biokompatibilitas. Nanomaterial berbasis karbon ini menunjukkan sifat penginderaan termal yang sensitif. Selain itu, peningkatan nanopartikel bebas logam penting dan mendesak karena bahaya lingkungan untuk aplikasi biologis untuk bahan beracun tersebut [51, 52]. Di antara keluarga nanomaterial berbasis karbon, nanodiamond fluoresen pertama kali dilaporkan sebagai nanotermometer [53]. Nanodiamond fluoresen memiliki biokompatibilitas intrinsik yang dihasilkan dari permukaan yang kuat dan lembam secara kimiawi [54]. Nanodiamond lain baru-baru ini digunakan untuk penginderaan termal intraseluler dengan akurasi sub-derajat [55, 56]. Sensitivitas termal dari nanodiamond ini didasarkan pada apa yang disebut pusat warna kekosongan nitrogen, yang merupakan cacat titik yang terdiri dari atom nitrogen yang menggantikan atom karbon kisi dan situs kisi kosong di dekatnya [48, 57]. Pusat kekosongan nitrogen dari nanodiamond fluoresen dipelajari secara ekstensif dan telah dikarakterisasi dengan baik untuk fotofisikanya, serta penggunaannya dalam aplikasi biologis [58]. Prinsip fungsi termometri berbasis kekosongan nitrogen tergantung pada pengukuran akurat dari pusat warna ini, yang dapat dideteksi secara optik dengan resolusi spasial yang tinggi [30, 59]. Namun, efisiensi fluoresensi rendah dan pengendalian yang buruk sangat menghambat penerapan nanodiamond fluoresen [36].

Salah satu kelas baru dari keluarga bahan nano berbasis karbon adalah titik karbon sangat bercahaya (CD), yang memiliki fotoluminesensi terang yang luar biasa, stabilitas fotokimia, kelarutan dalam air, biokompatibilitas yang besar, dan non-toksisitas [60,61,62]. CD adalah nanopartikel berdimensi nol dan berbentuk bola yang memiliki diameter kurang dari 10 nm [63, 64]. Berbagai pendekatan telah diterapkan untuk mempersiapkan beragam jenis CD, misalnya, ablasi laser [65], solvothermal [66], sintesis hidrotermal [67], bantuan gelombang mikro [68], pelepasan busur [69], oksidasi asam [70], dan lebih banyak pendekatan kimia dan fisik [71, 72]. Selain itu, CD menampilkan prospek yang diinginkan untuk berbagai aplikasi, seperti pencitraan biologis [73], kimia dan biosensing [74,75,76,77], pengiriman obat yang ditargetkan [78], analisis farmasi [79], dan katalisis [80, 81]. Titik karbon telah ditinjau secara ekstensif dalam hal sintesis, sifat fisikokimia, serta aplikasi potensialnya. Di sini, kami merujuk pembaca ke banyak ulasan bagus tentang titik karbon [72, 82,83,84,85,86,87,88].

Dalam beberapa tahun terakhir, CD fluorescent sebagai sensor suhu telah menarik banyak perhatian dari para peneliti. Pada prinsipnya, beberapa persyaratan diperlukan untuk pengukuran suhu yang efektif, seperti nanodot karbon harus menunjukkan variasi yang cukup besar dalam fotoluminesensinya pada kisaran suhu yang relevan [89]. Fotostabilitas, stabilitas pH, dan masa simpan adalah persyaratan lain yang harus dipertimbangkan untuk aplikasi praktis.

CD adalah alternatif yang menjanjikan untuk titik kuantum semikonduktor (SQDs) konvensional. Dibandingkan dengan QD, CD menunjukkan banyak keunggulan luar biasa seperti biaya rendah, toksisitas rendah, dan sifat optik/kimia uniknya yang kuat [90] Selain itu, CD menunjukkan lebih sedikit photobleaching. Dibandingkan dengan bahan baku fluorescent lainnya, CD diproduksi dari sumber karbon murah yang melimpah di alam [91]. Selanjutnya, ada beberapa metode sederhana untuk memodifikasi dan memfungsikan keadaan permukaan CD, yang memungkinkan penyesuaian kelarutan, stabilitas, sifat fisikokimia, dan hasil kuantum CD sesuai dengan persyaratan eksperimental mereka [49, 92, 93].

Dalam literatur, ada sejumlah artikel yang dilaporkan tentang titik karbon dengan fluoresensi yang bergantung pada suhu dan ditunjukkan pada Tabel 1.

Yu dkk. [51] pada tahun 2012 adalah yang pertama menyelidiki fluoresensi yang bergantung pada suhu dalam nanodot karbon dan membandingkannya dengan semikonduktor dan nanopartikel berbasis logam. Mereka bergantung pada pengukuran masa pakai fotoluminesensi yang bergantung pada suhu melalui teknik penghitungan foton tunggal yang berkorelasi dengan waktu (TCSPC). Dinamika relaksasi fotoluminesensi menjadi lebih cepat pada suhu yang lebih tinggi (Gbr. 2a), yang dapat dianggap berasal dari proses peluruhan nonradiatif. Pengukuran spektrum fluoresensi film CD sebagai fungsi suhu dari kriogenik ke suhu kamar dilakukan (Gbr. 2b). Dengan peningkatan suhu, intensitas fluoresensi menunjukkan penurunan berulang.

a Pengukuran fotoluminesensi yang diselesaikan dengan waktu sebagai fungsi suhu. b Spektrum fluoresensi film CD sebagai fungsi suhu. c Spektrum fluoresensi pada 300 K dilengkapi dengan fungsi dua Gaussian. d Bandwidth fluoresensi sebagai fungsi suhu. (Direproduksi dengan izin dari referensi [51])

Spektrum PL menunjukkan puncak asimetris, oleh karena itu spektrum PL pada setiap suhu dapat dipasang dengan baik oleh fungsi dua-Gaussian yang ditunjukkan pada Gambar. 2c. Pita energi tinggi, pita I; pita energi rendah, pita II (Gbr. 2d).

Selanjutnya, celah energi (bandwidth) fluoresensi diperiksa sebagai fungsi suhu (Gbr. 2d). Bandwidth total diucapkan oleh suhu-independen (hamburan elektron-elektron), dan suhu-tergantung (elektron-phonon dan permukaan/cacat hamburan). Bandwidth pita I dan pita II tidak bergantung pada suhu, yang menentukan bahwa hamburan elektron-elektron mendominasi dalam CD.

Oleh karena itu, efek suhu yang lemah dalam CD sesuai dengan fakta bahwa mekanisme interaksi utama melibatkan interaksi elektron-elektron daripada kopling elektron-fonon. Selain itu, pita PL yang lebar (> 100 nm) biasanya diamati bahkan pada suhu yang sangat rendah (77 K) disebut interaksi elektron-elektron yang kuat (Gbr. 2c). Hasil ini mirip dengan nanocluster logam dan berbeda dari QD semikonduktor. Oleh karena itu, Yu dan rekan kerjanya berspekulasi bahwa -elektron dalam CD dapat bertindak sama seperti elektron bebas dalam nanocluster logam.

Kalytchuk dkk. [89] mensintesis N, S-CD yang larut dalam air yang sangat bercahaya dengan perlakuan hidrotermal satu langkah asam sitrat dan l-sistein. Mereka mengumpulkan spektrum penyerapan kondisi tunak pada berbagai suhu untuk mengkarakterisasi sifat PL ketergantungan suhu dari CD. Spektrum serapan N, S-CD yang terdispersi dalam air pada suhu antara 10 dan 70 °C (meningkat 5 °C per langkah) ditunjukkan pada Gambar 3a. Tidak seperti nanokristal semikonduktor, posisi dan intensitas pita serapan tidak berubah dengan suhu. Hasil serupa sebelumnya dilaporkan untuk CD yang disintesis dengan perlakuan hidrotermal glukosa dengan adanya glutathione [1].

a Penyerapan yang bergantung pada suhu diambil dalam kisaran suhu dari 10 hingga 70 °C. b Plot warna yang dinormalisasi dari emisi PL yang bergantung pada suhu pada suhu antara 2 dan 80 °C dengan ukuran langkah 2 °C dan eksitasi pada 355 nm. Perubahan tergantung suhu yang sesuai pada puncak PL maksimum maks (c ), PL fwhm (d ), dan intensitas PL terintegrasi (e ). (Direproduksi dari referensi [89])

Dalam pekerjaan mereka, mereka menunjukkan plot warna spektrum PL dari N, S-CD yang diperoleh pada suhu mulai dari 2 hingga 80 °C dengan langkah 2 °C (Gbr. 3b). Peningkatan suhu mengurangi intensitas emisi PL sekitar faktor 2 tanpa terdeteksi pergeseran emisi PL [89]. Posisi emisi PL maksimum, lebar penuh PL pada setengah maksimum (fwhm), dan intensitas PL terintegrasi ditentukan secara kuantitatif pada suhu yang dipelajari, dengan memadatkan hasil masing-masing pada Gambar 3c–e.

Posisi puncak PL dari N, S-CD menunjukkan ketergantungan suhu yang lemah, tidak seperti kebanyakan nanokristal semikonduktor, yang celah pitanya berubah dengan suhu, mendorong pergeseran emisi PL. Selain itu, PL fwhm mereka hanya menunjukkan pelebaran yang tidak signifikan (1,4 ± 1 nm) pada rentang suhu yang sama (Gbr. 3d), menunjukkan bahwa puncak PL dari N, S-CD menunjukkan pelebaran termal yang dapat diabaikan. Untuk mengkarakterisasi proses relaksasi nonradiatif yang terjadi di CD, mereka menganalisis pendinginan intensitas PL terintegrasi sebagai fungsi suhu. Plot intensitas PL terintegrasi yang bergantung pada suhu untuk N, S-CD ditunjukkan pada Gambar 3e, dengan nilai yang dinormalisasi ke intensitas pada 2 °C, mengungkapkan bahwa intensitas menurun secara monoton selama rentang suhu yang dipelajari, dengan itu pada 80 °C menjadi kira-kira setengahnya pada 2 °C. Berdasarkan hasil ini, energi aktivasi pendinginan termal untuk CD pada suhu antara 2 dan 80 °C diperkirakan 17,0 ± 0,7 meV menggunakan rumus Arrhenius untuk N, S-CD, yang mendekati nilai yang dilaporkan oleh Yu dkk. [51].

Selain itu, Kalytchuk dkk. [89] meneliti dinamika emisi CD pada suhu yang berbeda. Gambar 4a-c menunjukkan ketergantungan suhu yang kuat dari emisi PL yang diselesaikan dengan waktu, menunjukkan data spektroskopi emisi yang diselesaikan dengan waktu untuk tiga suhu yang berbeda. Peta emisi PL transien dari CD diperoleh di wilayah spektral antara 375 dan 650 nm pada 2, 50, dan 80 ° C, seperti yang ditunjukkan pada Gambar. 4a-c. Ada penurunan yang jelas dalam peluruhan PL saat suhu meningkat, menunjukkan bahwa CD memiliki sifat yang memuaskan untuk penginderaan suhu berbasis seumur hidup PL. Penting untuk dicatat bahwa peluruhan eksponensial tunggal yang seragam secara spektral diamati di seluruh profil emisi titik-titik pada semua suhu yang dipelajari, menunjukkan bahwa rekombinasi terjadi melalui saluran yang sangat mirip dan sangat memancarkan di seluruh ansambel CD. Dinamika PL CD bisa dibilang kualitasnya yang paling menjanjikan terkait dengan aplikasi penginderaan suhu. Secara khusus, sensitivitas suhu CD menjadikannya nanotermometer seumur hidup PL.

Emisi PL yang bergantung pada suhu yang diselesaikan oleh waktu dari CD. Plot warna yang dinormalisasi menunjukkan peta emisi PL yang diselesaikan dengan waktu untuk CD pada a 2 °C, b 50 °C, dan c 80 °C. d Plot warna yang dinormalisasi dari intensitas PL yang diselesaikan waktu pada emisi PL maksimum (λem =421 nm) pada suhu antara 2 dan 80 °C. e Masa hidup PL yang diekstraksi diplot sebagai fungsi suhu dalam kisaran 2–80 °C. (Direproduksi dari referensi [89])

Variasi masa pakai PL CD pada suhu antara 2 dan 80 °C telah diselidiki secara menyeluruh; Gambar 4d, e menunjukkan data PL yang diselesaikan waktu yang dikumpulkan pada titik emisi PL maksimum sebagai fungsi dari suhu. Plot warna PL transien di seluruh rentang suhu yang dipelajari disajikan pada Gambar. 4d, menunjukkan bahwa peningkatan suhu menyebabkan pemendekan monoton dari peluruhan PL yang tampak. Semua kurva peluruhan yang direkam dipasang menggunakan fungsi eksponensial tunggal. Data tentang masa hidup yang diekstraksi disajikan pada Gambar. 4e. Saat suhu meningkat dari 2 hingga 80 °C, masa pakai PL menurun secara monoton dari 11,0 menjadi 5,3 ns. Rentang suhu di mana sensitivitas seumur hidup PL ini telah ditunjukkan (2–80 °C) mencakup rentang suhu yang relevan secara fisiologis dan suhu pengoperasian tipikal dari banyak perangkat elektronik. Sensitivitas pseudo-linear mutlak dari probe termal berbasis CD seumur hidup PL ini adalah 0,08 ns K ⁻¹ , dan sensitivitas relatif maksimumnya adalah 1,79% K ⁻¹ pada 62 °C. Kesesuaian eksponensial tunggal dari peluruhan PL dari nanoprobe luminescent berbasis CD di seluruh rentang suhu yang dipelajari menghasilkan parameter tunggal, masa pakai PL (τ), yang dapat langsung dikonversi ke unit suhu menggunakan kurva kalibrasi. Ini merupakan keuntungan penting dibandingkan titik kuantum semikonduktor tipikal, yang menunjukkan peluruhan multi-eksponensial yang membatasi kegunaannya dalam aplikasi yang melibatkan pengukuran masa pakai PL.

Mereka juga mempelajari ketergantungan suhu masa pakai CD PL dalam phosphate-buffered saline (PBS) dan medium Eagle yang dimodifikasi Dulbecco (DMEM) dan menunjukkan perilaku serupa.

Untuk mendemonstrasikan kegunaan kembali termometer pendaran berbasis CD, kurva peluruhan PL dari sampel yang dipilih diukur selama tujuh siklus pemanasan dan pendinginan berturut-turut pada suhu antara 15 dan 45 °C (Gbr. 5a).

a Plot warna yang dinormalisasi dari reversibilitas peluruhan PL selama tujuh siklus pemanasan dan pendinginan berurutan. b Stabilitas termal yang sesuai dari masa pakai PL selama tujuh siklus pemanasan dan pendinginan antara 15 dan 40 °C. (Direproduksi dari referensi [89])

Dalam setiap siklus pengukuran, peluruhan PL diukur setelah kesetimbangan termal 5 menit. Tidak ada histeresis termal yang diamati selama siklus pemanasan dan pendinginan, dan variasi masa pakai PL yang dihasilkan diplot sebagai fungsi waktu dalam (Gbr. 5b), menunjukkan bahwa masa pakai PL CD menunjukkan stabilitas termal yang sangat baik.

Kemudian, beberapa CD dengan emisi yang bergantung pada suhu disiapkan menggunakan berbagai metode sintesis, seperti perlakuan hidrotermal dan solvothermal [2, 77, 92, 95, 97,98,99,100,101,102,103], refluks panas [94, 96], dan laser ablasi [14] seperti yang ditunjukkan pada Tabel 1. CD yang disiapkan menunjukkan fluoresensi linier yang bergantung pada suhu pada rentang fisiologis (ditunjukkan pada Tabel 1). Intensitas fluoresensi CD menurun dengan meningkatnya suhu. Selanjutnya, reversibilitas dan pemulihan intensitas fluoresensi telah dipelajari di semua artikel. Gambar 6 menunjukkan beberapa sifat umum CD yang dipelajari di berbagai artikel.

a Foto digital eksitasi N, S-CD di bawah sinar UV (365 nm) pada suhu yang berbeda selama proses pemanasan (atas) dan pendinginan (bawah). (Direproduksi dari referensi [96]). b Spektrum emisi fluoresensi CD co-doping N, S diukur dalam kisaran 5–75 °C (atas ke bawah) ketika tereksitasi pada 340 nm, inset:regresi linier yang sesuai dari suhu versus Ln (F /B ₀ ). (Direproduksi dari referensi [97]). c FL/FL₀ - plot suhu MnOx-CD selama proses pendinginan dan pemanasan. (Direproduksi dari referensi [103]). d Ketergantungan suhu reversibel dari PL larutan CD. (Direproduksi dari referensi [102])

Nguyen dkk. [14] mensintesis titik karbon (CD) menggunakan ablasi laser femtosecond dari bubuk grafit dalam etilen-diamin. Gugus fungsi yang melimpah terbentuk di permukaan yang membentuk beberapa keadaan permukaan di situs permukaan dan menghasilkan multi-emisi CD. Mereka menyelidiki sensitivitas suhu CD yang bergantung pada fluoresensi menggunakan spektrum fluoresensi kondisi tunak. Spektrum emisi yang bergantung pada suhu dari CD pada eksitasi 320 nm ditunjukkan pada Gambar. 7a. Kedua intensitas fluoresensi puncak 400 dan 465 nm secara bertahap menurun dengan meningkatnya suhu karena aktivasi termal jalur peluruhan nonradiatif. Intensitas puncak berubah secara linier dengan suhu berkisar antara 5 hingga 85 °C (Gbr. 7b). CD yang peka terhadap suhu menunjukkan perubahan intensitas fluoresensi masing-masing 3,3 dan 2,1% per ° C untuk puncak 400 dan 465 nm. Hal ini menunjukkan bahwa CD dapat digunakan sebagai sensor suhu berbasis intensitas konvensional dengan sensitivitas tinggi.

a Spektrum emisi CD direkam dari 5 hingga 85 °C, tereksitasi pada 320 nm. b Intensitas fluoresensi dari puncak 400 dan 465 nm versus suhu. c Rasio 400 nm di atas puncak 465 nm sebagai fungsi suhu. d Studi reversibilitas suhu CD antara 20 dan 50 °C. (Direproduksi dari referensi [14])

Hebatnya, properti multi-emisi yang unik membuat CD menjanjikan fluorofor untuk sensor suhu fluoresen rasiometrik. Rasio dua intensitas fluoresensi pada 400 dan 465 nm (eksitasi 320 nm) versus suhu ditunjukkan pada Gambar. 5c. There is a very good linear relationship between the intensity ratio and temperature in a wide temperature range from 5 to 85 °C (R ² =0.998). Thermal linearity is advantageous since it makes the correlation between the peak-intensity ratio and temperature straightforward and meanwhile provides a constant thermal sensitivity along with the entire dynamic range. The temperature-sensitivity of CDs is determined to be 1.48% °C ⁻¹ , which is comparable with that of other materials. It should be noted that the temperature response range of CDs is much wider than those of other reports on dual-emission temperature sensors and covers both the physiological temperature for biology studies and the working temperature for many electronic devices. Besides 320 nm excitation, the CDs also work at other excitation wavelengths, such as 340 and 365 nm, with the same sensitivity. Thus, the CDs can be utilized for temperature sensing in many practical applications by selected different working wavelengths.

They have shown that the ratiometric temperature sensor was reversible between 20 and 50 °C, four cycles and photostable (when the intensity of the power source changed) as shown in Fig. 7d. This result suggests that the CDs sensing system is stable and robust with any changes in sample concentration, excitation, or detection efficiency.

Increasing temperature is not always accompanied by PL quenching; however, it could show enhancement of the PL as well. Macairan et al. [29] showed the PL enhancement of dual-fluorescent carbon dots with increasing temperature. They prepared biocompatible dual-fluorescing carbon dots CDs in a one-step microwave assisted-reaction using formamide and glutathione. They found that following excitation at 640 nm, the fluorescence intensity and PL integrated area increase over the range of 5–60 C by a factor of 3.5 observed over the entire analysis range and the temperature (Fig. 8a). As shown in Fig. 8b, a linear response (R ² =0.999) is observed over the entire analysis range and the temperature sensitivity was determined to be as high as 3.71% C ⁻¹ .

a Excitation at 640 nm yields a 3.5-fold increase in fluorescence intensity and the corresponding integrated area is plotted in b showing a linear response over the range of 5–60 °C. c Changes in the fluorescence spectra of the CDs (λ_ex =405 nm) as a function of temperature over the entire range. A 1.3-fold decrease is noted for the blue fluorescence in contrast to the 3-fold increase for the red counterpart. d The ratio of the integrated areas of the red and blue fluorescence components are plotted as a function of temperature showing a linear increase over the entire temperature range. (Reproduced from reference [29])

The temperature-dependent fluorescence was also studied following excitation at 405 nm. Interestingly, the blue and red fluorescence bands are not equally sensitive to the change in temperature. With increasing temperature, the fluorescence intensity (and the corresponding integrated area under the curve) of the blue component shows a very slight decrease in contrast to the red component, which significantly increases (Fig. 8c). These observations are noted over the range of 5–60 °C where the blue emission decreases by a factor of 1.3 in contrast to the red emission, which increases by a factor of 3.0.

As shown in Fig. 8d, the ratio of red to blue fluorescence increases with temperature, and a highly linear response is triplicate on 3 unique samples and the linear plot reflects the observed with an R ² =0.998. These analyses were repeated in an average of these measurements, which have small deviations at each temperature. The thermal sensitivity of the CDs, over the entire temperature range, varied from 1.33 to 4.81% °C ⁻¹ , which is an improvement over previously reported carbon dot nano-thermometry systems and other dual-emitting nanomaterials such as quantum dots and metal-organic frameworks-dye composites. The thermal resolution of the CDs was calculated to be 0.048 K ⁻¹ indicating that it is indeed possible to measure small thermal changes.

Zhang dkk. [98] synthesized CDs that have temperature-responsive characteristics in the range of 25–95 °C, and they have excellent sensitivity and remarkable reversibility/recoverability (Fig. 9a). CD/epoxy composites were further prepared by uniformly doping CDs into an epoxy resin. First, 5 μg of the CDs were dissolved in 50 μL of triethylenetetramine (TETA). Then, 350 μL epoxy resin was added to the mixed solution and fully mixed by high-speed stirring. The resulting composite showed significantly enhanced temperature response.

a Temperature dependence of the CD emission. b CD/epoxy composites. c Temperature dependence of the emission of the CD/epoxy composites. (Reproduced from reference [98])

Epoxy resin is a common thermosetting resin and is widely used to package LED chips. Figure 9b shows optical micrographs of CD/epoxy composite discs of approximately 2 cm in diameter and 8–10 mm in thickness. The cured CD/epoxy composites are transparent, and their fluorescence emission spectra are shown in Fig. 9c. The emission peak of the CD/epoxy composite is blue-shifted by approximately 10 nm compared to that of the CDs’ solution. Notably, the temperature response of the composite is significantly improved. In the temperature range of 25–95 °C, the fluorescence intensity decreases by 35% with increasing temperature, which is more than twice that of the solution state, and the linear results are more stable. The linear equation satisfies I ₀ /Aku =0.0074 [°C] + 0.80454 (R ² =0.99724), where I ₀ dan Aku are the fluorescence intensity of the CD/epoxy composite before and after the temperature rise, and the excitation wavelength is 360 nm. The blue shift of the emission peak and the enhancement of the temperature response characteristics may be due to changes in the dielectric constant of the environment in which the CDs are located. The composite has a wide temperature detection range, and its excellent sensitivity and stability make it suitable for use as a temperature sensor based on a fluorescent nanomaterial in a variety of environments.

Mechanism of Thermo-sensing

Up to now, there is no well-established mechanism for explaining the thermo-sensing behavior of carbon dots. Some reports attribute the mechanism to the thermal activation of non-radiative channels of surface (trap/defect) states. The general picture is that the non-radiative channels were not activated at low temperatures, so the excited electrons could emit photons radiatively. On the contrary, as the temperature increases, more non-radiative channels became activated, and excited electrons got back to the ground state by non-radiative processes, leading to the decreasing fluorescence intensity [2, 95, 99, 100, 103]. The mechanism of CDs emissions with heating/cooling is shown in Fig. 10.

Schematic illustration of CDs responding to temperature changes

To better understand the thermodynamics of the CD emission processes, Kalytchuk et al. [89] correlated the radiative (\( {\tau}_r^{-1} \)) and nonradiative ( \( {\tau}_{nr}^{-1} \)) recombination rates of a CD sample with its PL quantum yield. The radiative rate is determined from the PL quantum yield (QY) and the measured recombination rate τ ⁻¹ as \( {\tau}_r^{-1} \) =QY × τ ⁻¹ . The nonradiative relaxation rate \( {\tau}_{nr}^{-1} \) is expressed as \( {\tau}_{nr}^{-1} \) =τ ⁻¹ - \( {\tau}_r^{-1} \). The PL QY of CDs at various temperatures was calculated from their temperature-dependent absorption and integrated PL intensity together with the PL QY determined at room temperature. Both radiative and nonradiative recombination rates derived from time-resolved PL measurement data are plotted as functions of temperature in Fig. 11. The radiative recombination rate is greater than the correspondent nonradiative rate up to 70 °C and does not vary appreciably at temperatures between 2 and 80 °C, remaining in the range of (0.74–0.82) × 10 ⁶ s ⁻¹ . In contrast, there is a pronounced (almost 7-fold, from 0.16 × 10 ⁶ to 1.12 × 10 ⁶ s ⁻¹ ) monotonic increase in the rate of nonradiative recombination by increasing the temperature from 2 to 80 °C. Temperature-dependent crossover of the radiative and nonradiative rates occurs at 70 °C, at which temperature the PL QY is 50%. These results suggest that the temperature activation of PL quenching in their CDs is primarily caused by the activation of nonradiative relaxation channels [89].

Radiative (solid symbols, blue color) and nonradiative (hollow symbols, red color) recombination rates for CDs plotted against the temperature for temperatures of 2–80 °C. (Reproduced from supporting information of reference [89])

Guo dkk. [102] ascribed the thermal-quenching of their prepared carbon dots not just to activation of the nonradiative decay process, but also to the occurrence of nonradiative trapping with increasing temperature. They measured the temperature-dependent decay lifetimes of the CDs and shown in Fig. 12a. The data were collected by monitoring emission maximum as a function of the temperature under the 320 nm laser excitation. The result shows that the PL lifetime drops from 15.03 to 11.70 ns with the temperature increasing from 283 to 343 K, which could be ascribed to the occurrence of nonradiative decay processes. Besides, the PL relaxation dynamics of the CDs reveal multi-exponential decay with temperature increasing, which suggests the photoexcited carriers following the complicated relaxation processes. The occurrence of non-radiative trapping will be increased with rising temperature, and this could be quantitatively analyzed by the Arrhenius plot of the integrated PL intensities as:

$$ I={I}_0/\left[1+\mathrm{a}\ \exp\ \left(\hbox{-} {\mathrm{E}}_a/\mathrm{kT}\right)\right] $$

Temperature-dependent decay curves of CDs solution (a , λ_ex =350 nm, λ_em =450 nm); the dependence of ln[(I ₀ /Aku _B )-1] on 1/kT CDs solution (b ). (Reproduced from reference [102])

dimana E _a is the activation energy, k is the Boltzmann constant, and a adalah sebuah konstanta. Figure 12a displays the plotting of the emission intensity with respect to 1/T , where the value of activation energy (E _a ) is calculated to be 0.329 ± 0.02 eV. In order to probe the reason for thermal quenching of CDs emission process, the radiative (V _r ) and nonradiative (V _nr ) recombination rates of CDs were determined from the lifetime (τ*) and quantum yield (QY) as:

$$ {\tau}^{\ast }=\frac{1}{V\mathrm{r}+V\mathrm{nr}};\mathrm{QY}=\frac{V\mathrm{r}}{V\mathrm{r}+V\mathrm{nr}} $$

The QY of CDs at various temperatures was calculated from their temperature-dependent absorption and integrated PL intensity with the QY determined at room temperature. They have noticed that the radiative rates have a slight decline when the temperature rises from 283 to 343 K; at the same time, the nonradiative recombination rates have gradually increased by about 2-fold. These results further indicate that the temperature-activated PL quenching in CDs is mainly due to the activation of nonradiative relaxation channels [102].

Other groups used microscopic and spectroscopic techniques to understand the mechanism of thermos-sensing of carbon dots.

Wang at el. used TEM and UV–Vis spectra to study the temperature-responsive PL behavior of prepared CDs. As shown in Fig. 13a, the CDs display no change in the UV–Vis spectra upon increasing the temperature from 20 to 80 °C. However, it was found that the average diameter of CDs increased from 2.6 ± 0.2 nm at room temperature to 4.4 ± 0.2 nm at 80 °C (Fig. 13b). Thus, increasing the temperature, the aggregation of as-prepared CDs occurred which caused the obvious fluorescence quenching [1].

a UV–Vis absorption spectra of CDs in aqueous solution under 20 and 80 °C. b the TEM image of CDs in aqueous solution (a ) at room temperature, the average size was 2.6 ± 0.2 nm (b ) at 80 °C, and the size increased up to 4.4 ± 0.2 nm. (Reproduced from reference [1])

Dia dkk. [101] reported that the hydration particle size of their CDs emerges as larger with the increase in the temperature (Fig. 14a), which indicates that the temperature rise gives rise to the aggregation of the CDs, eventually results in the fluorescence quenching. Nonetheless, with the decline in the temperature, the hydration particle size of CDs starts declining (Fig. 14b), which indicates that the cooling has the potential of causing CDs to depolymerize [101].

Change of hydrated particle size of carbon dots during heating (a ) and cooling (b ). (Reproduced from reference [101])

Another group such as Cui et al. also attributed the fluorescence quenching to the aggregation of CDs. They also tried to apply the undoped CDs synthesized using only acrylic acid as a precursor in temperature sensors. Unfortunately, undoped CDs possessed weaker quenching effects under the same temperature elevation than doped CDs [92].

Yang dkk. [94] in their work proposed two key factors concerning the temperature-dependent PL property of the N-CDs, including (i) surface functional groups and (ii) hydrogen-bonding interaction. To examine the effect of the first factor, the surface O-containing groups, another control experiment was conducted by treating N-CDs (4.0 mL) with a strong reducing agent NaBH₄ (1.0 mL, 0.1 mol L ⁻¹ ) to remove C=O species on carbon dots surface. The obtained reduced N-CDs are denoted as r-N-CDs for brevity. Compared with N-CDs, the r-N-CDs exhibit weaker fluorescence intensity (Fig. 15a). Besides, the fluorescence intensity of r-N-CDs only decreases by 13% with temperature increasing from 20 to 80 °C (the inset in Fig. 15a) that gives a much lower temperature sensitivity, which is ascribed to the decreased O-containing groups [94].

a PL spectra (excitation wavelength, 400 nm) of N-CDs (black trace) and r-N-CDs (red trace). The inset shows I /Aku ₀ −T of reduced N-CDs. b PL spectra (excitation wavelength, 400 nm) of N-CDs dispersed in C₂ H₅ OH at various temperatures. c A schematic mechanism for the temperature-dependent fluorescence intensity of N-CDs. (Reproduced from reference [94])

The second factor, the effect of hydrogen bonding with the solvent on fluorescent behavior of N-CDs was explored. N-CDs solution (1.0 mL) was dropped on a filter paper and left to dry in the air to obtain a solid sample, which still emits bright fluorescence. However, no obvious change of fluorescence intensity of the solid N-CDs with temperature increase was observed. They also measured the fluorescence of N-CDs dispersed in ethanol. The fluorescence intensity of N-CDs in C₂ H₅ OH is lower than that in water and little variation of the PL intensity is observed with temperature increasing from 20 to 80 °C (Fig. 15b). Hence, the strong hydrogen bonds play a key role in the temperature-dependent PL property of the N-CDs. Figure 15c is a schematic mechanism for the temperature-dependent fluorescence intensity of N-CDs [94].

However, our group used the same experimental strategy as Yang group; in both cases, the r-CDs and e-CDs emissions were quenched linearly with increasing temperature, in the same way as the original results of their CDs (Fig. 16). Thus, our results ruled out the synergistic effects of abundant oxygen-containing functional groups and hydrogen bonds [77].

a , b Fluorescence spectra of reduced CDs (r-CDs) and CDs in ethanol (e-CDs) at temperatures (20 to 60 °C). c , d Linear correlation between fluorescence intensity and temperature (°C) for r-CDs and e-CDs respectively. (Reproduced from the supplementary information of reference [77])

Bioimaging in Living Cells (Thermal Imaging)

In literature, only a few articles explored the temperature-responsive fluorescent properties of CDs in biological imaging. Prior to such experimentations, in vitro cytotoxicity analysis is crucial for CDs because they make it possible to estimate the CD’s toxicity in living subjects. In vitro cytotoxicity analysis evaluates the effect or influence of the nanomaterial on cultured cells [89].

Yang dkk. [94] verified that the CDs could be used as an effective thermometer in living cells; HeLa cells were washed with PBS after treatment with CDs for 6 h. Bright-blue fluorescence of the CDs in HeLa cells is observed when the temperature is 25 °C as shown in Fig. 17a. With the temperature increasing to 37 °C, the blue fluorescence becomes weaker (Fig. 17b), the fluorescence of N-CDs is recovered when the temperature was decreased to 25 °C (Fig. 17c). As a thermos-imaging in vivo, the fluorescent images of mice were collected immediately after being injected with N-CDs at different temperatures. By setting the fluorescence intensity at 28 °C as the reference (I _o ), Aku /Aku _o of the area where CDs were injected varies from 1.0 to 0.87 with the increase of temperature from 28 to 34 °C (Fig. 17d, e). With temperature further increasing to 43 °C, I /Aku _o declines to 0.52 and the fluorescence becomes nearly undetectable (Fig. 17f). And I /Aku _o can be reversibly enhanced back from 0.66 to 1.0 with the temperature decreases from 39 to 28 °C (Fig. 17g–i). All of these results indicated that N-CD could be used as an effective in vitro and in vivo nanothermometer [94].

a–c Fluorescent images of a single Hela cell at 25, 37, and 25 °C after treatment with N-CDs, respectively. d –f Fluorescent photographs of a mouse given an injection of N-CDs at increasing temperatures. g –i Fluorescent photographs of a mouse given an injection of N-CDs at decreasing temperatures. ( Reproduced from reference [94])

In an exploratory experiment, Kalytchuk et al. tested the capacity of CDs for intracellular temperature monitoring in human cervical cancer HeLa cells. Figure 18 shows the measured intracellular temperatures of HeLa cells incubated with CDs (500 μg/mL). The CDs’ PL decay curves at each temperature were highly reproducible and could be fitted with a single-exponential function at all recorded temperatures. In Fig. 18a, the recorded PL decay curves for temperatures between 25 and 50 °C (with a step size of 5 °C) are indicated by symbols, while the corresponding single-exponential fits are represented by solid lines. They were able to confirm that the PL signal in these PL lifetime measurements was derived exclusively from the CDs for the CD concentration of ≥ 100 μg/mL. The intracellular temperature in each measurement was determined from the calibration curve between PL lifetimes as the temperature increases from 2 to 80 °C. The temperatures determined in this way (T _meas ) are plotted as functions of the PL lifetime in Fig. 18b. Independently, the temperature of the cell solution was determined using a calibrated reference thermometer (shown in Fig. 18b as T_set ). The temperatures reported by the luminescent CD probe and the reference detector are in good agreement. These results show that the PL lifetime of nanoprobes based on CDs can be reliably used to measure intracellular temperatures [89].

In vitro intracellular PL lifetime thermal sensing using CDs. a PL emission decays of HeLa cells incubated with CDs (500 μg/mL) at different temperatures (T _set ). b Temperatures determined using the calibration (T _meas ) and set temperatures (T _set ) plotted against the PL lifetime. c –f Applicability of CDs for long-term remote intracellular temperature monitoring. c PL lifetimes extracted from PL transients recorded every 15 min for 24 h of HeLa cells incubated with CDs (500 μg/mL). d Temperatures determined using the calibration curve. e Temperatures measured with a reference thermometer (T _ref ). f Histogram showing the distribution of temperature differences between the obtained and reference temperatures; the solid line is the distribution curve. (Reproduced from reference [89])

To further evaluate the potential of luminescent CD nanoprobes for long-term real-time temperature monitoring, PL decay profiles of HeLa cells incubated with CDs (500 μg/mL) every 15 min for 24 h were recorded. The PL lifetimes extracted from the measured PL decay values during this period were then plotted as functions of time, as shown in Fig. 18c. Using these results, the temperature variation over time was calculated using the calibration curve, as shown in Fig. 18d. In addition, the sample’s temperature at each measurement point was determined using a reference thermometer with a temperature reproducibility of 60 mK. The temperature determined with the reference thermometer is plotted in Fig. 18e, which shows that there was excellent agreement between the measured and reference temperatures. The high accuracy of the PL-based temperature measurements is further demonstrated by statistical analysis of the differences between the measured and real (reference) temperatures (Fig. 18f). Using these data, the absolute average accuracy of temperature detection by the presented method was calculated to be 0.27 °C. This experiment confirms the potential of CD-based thermal probes in biological systems [89].

Macairan et al. [29] displayed that the prepared CDs can be used for thermal sensing inside cells using intensity and ratiometric approaches. HeLa cells treated with CDs were allowed to equilibrate at 32, 37, and 42 °C (Fig. 19). Excitation at 640 nm was used to selectively monitor the red fluorescence of the CDs in the cells. The thermal changes could be due to changes in intracellular concentration or not correlate with a change in intensity (λex + 640 nm). This could be due to changes in intracellular concentration or localization of the CDs at higher temperatures. Thus, simply relying on changes in fluorescence intensity leads to accurate intracellular thermal sensing.

Fluorescence microscopy images of CD-treated HeLa cells. Fluorescence signals from the CDs (λ_ex =640 nm; left and 405 nm; right) fluorescence ratios are 1.8 at 32 °C, 2.0 at 37 °C, and 2.3 at 42 °C. The control shows untreated HeLa cells at 42 °C with no fluorescence signal as expected. (Reproduced from reference [29])

In contrast, these limitations are excluded using the ratiometric approach. The CDs maintain dual blue and red fluorescence in cells following excitation at 405 nm, as previously observed for the colloidal dispersions (Fig. 19). The red-to-blue ratio increases with increasing temperature, with values of 1.8 at 32 °C, 2.0 at 37 °C, and 2.3 at 42 °C. The ratiometric relationship of the red-to-blue fluorescence of the CDs highlights the advantage of ratiometric temperature sensing in the development of fluorescent nano-thermometry probes. The relative red-to-blue emission ratio remains unaffected, regardless of the amount of CDs taken up by the cells, which can be affected by various factors such as confluency and is not concentration-dependent. Lastly, the CDs have shown fluorescence reversibility with respect to changes in intracellular temperature. Following incubation in HeLa cells, they were subjected to a heating/cooling cycle from 32/42/32 °C. This emphasizes the robustness of the proposed CD-nanothermometer and these findings further demonstrate the fluorescence reversibility [29].

Confocal laser scanning microscopy was used to thermal image colon cancer cell HT-29 using N, S, and I-doped CDs, as shown in Fig. 20 [100]. The fluorescent spots were temperature-dependent as shown in Fig. 20g–i, as the most intense is at 15 °C, while the weakest point was at 35 °C. Interestingly, the fluorescence spots were reversible, and the spots were very photostable after 20 min of continuous excitation [100]. Shin et al. [96] also used confocal laser scanning microscopy for Hela cells, shown in Fig. 20e–g.

a–d Are confocal microscopy images of N, S, I-CDs-colon cancer cell HT-29 with corresponding fluorescence field at 15, 25, 35, and 15 °C, respectively. (Reproduced from reference [100]). e –g Confocal microscopy images of N, S-CDs-stained cells with corresponding fluorescence field at 25, 35, and 25 1C, respectively. (Reproduced from reference [96])

Li dkk. 103 prepared a nanocomposite composed of MnOx-CDs to be used as a nano thermos responsive fluorophore for biological environments. HepG2 cells were incubated with MnOx-CDs at different culture temperatures. As illustrated by confocal laser scanning microscopy (excited at 405 nm), the blue luminescence of the MnOx-CDs in the HepG2 cells is weak when the environmental temperature was 40 °C (Fig. 21a), and the blue luminescence of the MnOx-CDs in HepG2 cells enhances as the temperature decreased to 30 °C (Fig. 21b) even to 20 °C (Fig. 21c). Due to the temperature-responsive properties, the as-synthesized MnOx-CDs can be readily applied in the biomedical fields like bioimaging and photothermal therapy in cancer treatment [103].

Confocal laser scanning microscopy images (excited at 405 nm) of MnOx-CDs in HepG2 cells at 40 °C (a ), at 30 °C (b ), and 20 °C (c ). (Reproduced from reference [103])

Conclusions and Future Perspective

Carbon nanodots exhibit unique properties to be exploited for nanothermometry, such as thermal-sensitivity, low-cost, and photostability. Flexible surface modification and facile preparation will pave the way to establish an enormous number of thermal sensitive nanomaterials for a variety of applications. The overall trends in thermo-sensing nanomaterials are aimed at enhancing photostability and thermal-resolution with using low-cost and safe materials. CDs can be classified as a new generation of thermometer that can fulfill these requirements and can be used for biomedical thermometry applications, such as temperature monitoring during hyperthermia treatment. Facile-preparation protocols, biocompatibility, and easy functionalization of CDs are promising criteria which make the CDs alternative next-generation nanothermometer materials. More efforts are required to promote basic research in this field. Limitations should be overcome to produce carbon dot-based nanothermometers comprising enhancing thermal sensitivity, and working in a broader range of temperature. A better understanding of the fluorescence thermal-sensing mechanism is another key issue to be able to design and manipulate the structure of CDs and enhance thermal resolution. More experiments and theoretical modeling are necessary to understand the correlation between the methods of fabrication of CDs with their thermal behavior.

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