Au@Ag Core@Shell Nanoparticles Disintesis dengan Rumex hymenosepalus sebagai Agen Antimikroba

Hasil dan Diskusi

Karakterisasi

Gambar 1a menunjukkan spektrum serapan UV–vis dari bahan nano yang disintesis. Absorbansi telah dinormalisasi untuk resonansi plasmon permukaan lokal maksimum (LSPR) yang sesuai untuk setiap sistem partikel nano.

Spektra UV–Vis nanopartikel (a ), Z-Potensial (b ), DLS (c ), dan PDI (d ) dari Au, Ag dan Au@AgNPs

Spektrum absorpsi Au@AgNps pada Gambar 1a memiliki pita tunggal yang berpusat pada 474 nm, yang terletak di antara AgNPs LSPR (445 nm) dan AuNPs LSPR (544 nm). Tidak adanya puncak penyerapan seperti emas pada Au@AgNPs menunjukkan bahwa bahan nano yang diperoleh dengan sintesis berurutan adalah struktur inti@kulit. Tidak mungkin mendeteksi pita serapan apa pun yang terkait dengan Au milik nukleus. Beberapa penulis berasumsi bahwa untuk sistem inti@kulit, spektrum serapan terdiri dari dua pita yang terkait dengan masing-masing logam untuk ketebalan cangkang antara 3 dan 4 nm. Penyerapan yang terkait dengan inti logam menghilang untuk ketebalan yang lebih tinggi, memperoleh pita serapan tunggal di mana lokasi maksimum tergantung pada rasio ketebalan/ukuran inti dari partikel bimetal [79, 80].

Samal dkk. [81] mensintesis inti@kulit nanopartikel (Au@Ag) dengan mengontrol ukuran inti dan menambahkan cangkang dengan ketebalan yang berbeda. Secara khusus, hasil UV-vis kami untuk Au@AgNPs bertepatan dengan yang dilaporkan oleh Samal et al. untuk inti emas 32 nm dan ketebalan perak lebih besar dari 15 nm, di mana spektrum dicirikan oleh pita serapan tunggal (~ 450 nm), dan penekanan plasmon permukaan Au diamati.

Selain itu, pada Gambar 1a, penyerapan yang berpusat pada 280 nm (wilayah yang disorot dengan warna biru) dapat diamati, sesuai dengan molekul dari Rumex hymenosepalus ekstrak digunakan sebagai agen pereduksi dalam sintesis nanopartikel kami. File tambahan 1:Gambar S1 sesuai dengan Rumex hymenosepalus spektrum absorpsi larutan air. Sebuah pita karakteristik berpusat pada 278 nm diamati, terkait dengan transisi elektronik dari cincin aromatik terkonjugasi dengan gugus karbonil senyawa polifenol [82]. Pita serapan ini dalam spektrum nanopartikel UV-Vis menunjukkan bahwa produk akhir mengandung molekul ekstrak yang tertinggal di dalamnya, Rivero-Cruz et. Al. telah melaporkan empat stilbenoid, dua flavan-3-ol, dan tiga antrakuinon yang diisolasi dari R. selaput dara [83] dan Rodríguez-León et. Al. telah melakukan studi resonansi magnetik nuklir untuk menentukan bahwa Rumex hymenosepalus mengandung molekul penting seperti stilbene glikosida dan epicatechin gallate dan epigallocatechin gallate [61]. Molekul-molekul ini berpartisipasi sebagai agen pereduksi dalam sintesis nanopartikel. Proses ini melibatkan deprotonasi beberapa gugus -OH dari cincin fenolik untuk membentuk gugus  = CO. Molekul polifenol dioksidasi, dan elektron yang dilepaskan ditransfer ke Ag ⁺ dan Ag ³⁺ ion untuk membentuk Au ⁰ dan Ag ⁰ .

Gambar 1b menunjukkan nilai potensial Zeta yang sesuai dengan AuNP, AgNP, dan Au@AgNPs pada konsentrasi 1, 10, 50, dan 100 µg/mL. Nanopartikel didispersikan dalam air ultra murni, dan pengukuran dilakukan dengan rangkap tiga pada 25 . Secara umum, di seluruh rentang konsentrasi, nanopartikel menunjukkan nilai potensial zeta lebih negatif dari -30 mV, mencapai nilai -40 mV pada konsentrasi 100 µg/mL untuk AuNPs dan Au@AgNPs dan -38 mV untuk AgNPs. Nilai potensial Zeta yang sangat negatif ini menunjukkan bahwa nanopartikel mengalami interaksi tolak-menolak di antara mereka yang mencegah agregasinya dan memungkinkan stabilitas koloid logam dalam jangka panjang [84,85,86]. Dengan mengkorelasikan hasil spektroskopi UV-Vis dengan nilai potensial Zeta yang diperoleh, kami dapat menetapkan bahwa nilai yang sangat negatif mungkin disebabkan oleh kompleksnya molekul polifenol ekstrak ke permukaan nanopartikel [87].

Untuk aplikasi biologis, penting untuk mendapatkan populasi nanopartikel dengan ukuran monodispersi [88], jadi Gambar 1c dan d menunjukkan nilai DLS yang sesuai dengan diameter rata-rata dan indeks polidispersitas (PDI) untuk AuNPs, AgNPs, dan Au@AgNPs pada konsentrasi 1, 10, 50, dan 100 µg/mL. Diameter rata-rata Au@AgNPs adalah sekitar 250 nm dan dijaga konstan sebagai fungsi konsentrasi, dengan cara yang sama untuk nanopartikel monometalik dengan diameter rata-rata masing-masing sekitar 122 dan 135 nm untuk AuNP dan AgNP. Dalam kasus yang sama amati bahwa nilai PDI adalah sekitar 0,3 untuk nanopartikel monometalik dan 0,2 untuk Au@AgNPs. Hasil ini, di mana ukuran tidak bervariasi dengan konsentrasi, menunjukkan bahwa sistem nanopartikel yang berbeda memiliki stabilitas yang baik menyajikan polidispersitas moderat ukuran (0,3 ≥ PDI ≥ 0.2).

Celah pita optik dari plot Tauc dihitung (File tambahan 1:Gambar S2). Pita konduksi oleh bahan semikonduktor memiliki nilai E _g < 3 eV, sebagai referensi germanium memiliki E _g < 0,7 eV dan silikon adalah E _g = 1.1 eV [89]. Dalam kasus kami, celah pita untuk Au@AgNPs, AuNPs, dan AgNPs masing-masing adalah 1,93 eV, 2,03 eV, dan 2,33 eV. Artinya bahan ini dianggap seperti semikonduktor, dan fitur ini diperoleh untuk efek kurungan kuantum yang menghasilkan celah energi yang meningkat, sehingga bahan nano dapat digunakan sebagai sensor, baterai, dan perangkat optoelektronik [64].

Untuk menentukan apakah senyawa organik hadir dalam nanopartikel, nanomaterial ini dicirikan oleh XPS dan FTIR. Gambar 2a menunjukkan spektrum survei ekstrak tumbuhan dan nanopartikel. Seperti dapat dilihat, semua sistem nanopartikel menampilkan sinyal karakteristik yang terkait dengan elemen karbon (C 1 s, 284,6 eV) dan oksigen (O 1 s, 532,2 eV) dari Rumex hymenosepalus ekstrak. Hasil ini menegaskan bahwa molekul organik dari ekstrak hadir dalam nanomaterial yang diperoleh. Selain itu, spektrum XPS resolusi tinggi diperoleh untuk menganalisis keadaan oksidasi perak dan emas dalam nanopartikel (File tambahan 1:Gambar S3). Untuk AgNP (File tambahan 1:Gambar S3A), sinyal pada energi ikat 373,76 eV dan 367,76 eV (∆BE = 6.0 eV) yang sesuai dengan garis spektral 3d3/2 dan 3d5/2 dapat dikaitkan dengan Ag ⁰ (perak metalik). Untuk AuNPs (File tambahan 1:Gambar S3B), puncak yang terkait dengan kopling spin-orbital 4f5/2 terletak pada energi ikat 87,65 eV. Untuk 4f7/2, puncak kopling spin-orbital terletak pada 83,98 eV. Rasio intensitas (I4f7/2 > I4f5/2), lokasi, dan pemisahan antara puncak (ΔBE = 3.67 eV) menegaskan bahwa ion emas (Au ³⁺ ) direduksi sepenuhnya menjadi emas metalik Au ⁰ [90]. Untuk Au@AgNPs, spektrum XPS resolusi tinggi yang sesuai dengan perak dan emas masing-masing ditunjukkan dalam File tambahan 1:Gambar S3C dan Gambar S3-D. Spektrum ini menunjukkan perilaku yang sama dengan rekan-rekan monometalik mereka. Ini menunjukkan bahwa baik perak maupun emas bervalensi nol dalam presentasi core@shell.

Spektrum survei XPS (a ), dan FTIR (b ) untuk Rh, Ag, Au, dan Au@AgNPs

FTIR pada Gambar. 2e menunjukkan pada 3296 cm ⁻¹ gugus hidroksil, 2974 cm ⁻¹ (C–H) ikatan cincin aromatik, 1689 cm ⁻¹ terkait dengan peregangan gugus karbonil (C=O), dan 1689–1400 cm ⁻¹ karena ikatan karboksilat (C–O) dan regangan ikatan karbon-karbon dan 1212 cm ⁻¹ terkait dengan regangan fenol C–O terkonjugasi dengan AuNPs, 1049 cm ⁻¹ Mode peregangan C–O dari molekul gugus katekin ester [88,89,90] dan 799 cm ⁻¹ pembengkokan bidang dalam fenol, pembengkokan C–H dilaporkan pada 964,4, dan 829,39 cm ⁻¹ fitur resveratrol [91] bergeser dalam ekstrak Rh ke 1031 dan 867 cm ⁻¹ , masing-masing, dan AuNPs, AgNPs, dan Au@AgNPs juga digeser yang menunjukkan kompleksasi molekul polifenol ekstrak Rh dengan nanopartikel.

Gambar 3a sesuai dengan mikrograf STEM medan terang representatif dari sistem Au@AgNPs pada perbesaran rendah (bilah skala 100 nm). A set of nanoparticles without agglomeration and with mostly quasi-spherical geometry can be seen. The same region is shown in dark field (HAADF) in Fig. 3b, and the core@shell structure can be observed, where can we distinguish Au-core looks more intense than Ag-shell, due to the difference in atomic number. Figure 3c and d corresponds to STEM higher magnification micrograph (scale bare 20 nm) of a nanoparticles group of system core@shell in a bright and dark field, respectively. Can be appreciated with clarity brilliant Au core and Ag shell lightly contrasted. These images show that the thickness of the Ag-shell varies between 3 and 5 nm. Additional file 1:Figure S4 corresponds to an individual images gallery where can be observed uniformity of Ag-shell.

Scanning Transmission Electron Microscopy at low magnification (scale bar 100 nm) of Au@AgNPs in Bright Field (a ) and HAADF (b ). A Small group of Au@AgNPs at higher magnification (scale bar 20 nm) in Bright Field (c ) and HAADF (d )

Figure 4a, c, and e corresponds to micrograph TEM of representative nanoparticles systems AuNPs, AgNPs, and Au@AgNPs, respectively. In all cases, nanoparticles have sphere-like morphology and are shown well separated from each other. This can be explained by the extract molecules onto nanoparticle surfaces, acting as spacers between them. Figure 4b, d, and f shows histograms corresponding to size distribution obtained by TEM and performed with 500 nanoparticles collected from 15 to 20 micrographs for each nanomaterial. The histogram presents Gaussian distribution with a mean size of 24 ± 4 nm (AuNPs), 13 ± 3 nm (AgNPs), and 36 ± 11 nm (Au@AgNPs). The discrepancy in values between DLS and TEM measurements with size distribution graph is due to conditions micro-environmental around the nanoparticles, while DLS shows a diameter for a system that includes metal, hydrated coating, and solvent by comparison in TEM measurements is a dry system where measurement is over metal only, in particular, in DLS the molecules used for complexation of nanoparticles (reducing agents and stabilizers) are dispersants that induce errors in sizing measurement and shifts it results to higher values [91].

TEM and size distribution of AuNPs in (a ) dan (b ), AgNPs in (c ) dan (d ), and Au@AgNPs in (e ) dan (f )

Figure 5 corresponds to the Au@AgNPs HAADF-STEM micrographics. A single nanoparticle is shown in Fig. 5a with a gold nucleus and silver cover perfectly delimited, the atomic number (Z) changes through the interface Au@Ag, intensity variations can be quantified by HAADF-STEM [67].

Au@AgNPs HRTEM (a ). Magnification from (a ) of interface core–shell (b ). FFT plot with Miller index (c ) and integrated image from FFT (Inverse) with interplanar distance (d )

The red square region is amplified to obtain an HRTEM micrography of the shell portion (Fig. 5b), and then to verify the crystalline shell structure, the nanoparticle periphery region was analyzed (discontinued square) with the Digital Micrograph 3.0 software (Gatan). Fast Fourier Transform (FFT) image of the selected area was obtained (Fig. 5c). Using the Inverse Fast Fourier Transform was possible to estimate interplanar distances of 2.3 Å, 2.0 Å, and 1.4 Å in Fig. 5d. These distances can be assigned, respectively, to the crystalline planes (111), (200), and (220) of face-centered cubic (fcc) silver according to Inorganic Crystal Structure Database (ICSD) at the FIZ Karlsruhe–Leibniz Institute for Information Infrastructure, Germany or the electron crystallography software Jems (V 4-5430, JEMS-SAAS, Switzerland) [92]. A similar analysis of crystal structure by HRTEM was carried out for monometallic nanoparticles as illustrated in Additional file 1:Figures S5 (for AuNPs) and S6 (AgNPs). In both cases, crystal structure corresponds to face-centered cubic (fcc).

EDS chemical analysis shows the presence of both metals for a group of bimetallic Au@AgNPs observed by TEM (Fig. 6a) in proportions of the atomic weight percent 77% of Ag (shell) and 23% of Au (cores) (Fig. 6b). In comparison, a single bimetallic (Fig. 6c) Au@AgNPs has proportions around 80% of Ag (shell) and 20% of Au (core) (Fig. 6d). To estimate the gold and silver atomic percentage on core@shell nanoparticles (Au@Ag), a quasi-spherical geometry approximation of nanoparticles morphology was considered. The AuNPs average diameter obtained from TEM size distribution (\(\overline{D}_{{{\text{Au}}}} =24\;{\text{nm}}\)) was used for core volume estimation (V _Au ), and Au@AgNPs average diameter \(\left( {\overline{D}_{{{\text{Au}}@{\text{Ag}}}} =36\,{\text{nm}}} \right)\) for core@shell volume estimation \((V_{{{\text{Au}}@{\text{Ag}}}} )\). So, shell volume is determined as \(V_{{{\text{Ag}}}} =V_{{{\text{Au}}@{\text{Ag}}}} - V_{{{\text{Au}}}}\). Total atomic content of Au and Ag was calculated considering an fcc crystalline structure (4 atoms per unit cell) for booth metals where the Au and Ag lattice parameters are 4.0783 Å and 4.0862 Å, respectively [93]. Atomic content estimation by this procedure is 70% for Ag (Shell) and 30% Au (Core), which differs by 10% concerning the measurement obtained by EDS. Figure 7 shows a theoretical estimation of silver and gold content and how varies as the thickness of the shell increases and the size of the core is kept constant (\(\overline{D}_{{{\text{Au}}}} =24\;{\text{nm}}\)). It is observed that for Au@AgNPs with a diameter greater than 30.2 nm, the atomic content of silver exceeds the content of gold. In Supplementary Material, a detailed description of calculation is carried out to obtain atomic contents percentage. Similar estimations of atomic percentages were carried out considering other Au-core diameters and varying Au@AgNPs diameter (Additional file 1:Figure S7A-B).

TEM and EDS of an Au@AgNPs group (a , b ) with Au 23 and Ag 77% at. and single Au@AgNPs (c , d ) with Au 20 and Ag 80% at

Variations of atomic content percentage versus increase thickness shell, where diameter AuNPs (24 nm) was kept constant

Figure 8 corresponds to XRD patterns for AuNPs and AgNPs as well as bimetallic Au@AgNPs. All the synthesized products have fcc crystalline structure as previously reported in the characterization by electron microscopy. Peaks for Au@AgNPs are located at 2θ diffraction angles of 38.25°, 44.4°, 64.9°, 77.85°, and 81.25°. As can be appreciated in the figure, the AgNPs and AuNPs diffraction peaks are found in the same positions mentioned with a difference of ± 0.5°. This is because Au and Ag have very similar lattice constants, so their diffraction patterns for fcc crystal structure are almost identical [94,95,96]. In this way, the diffraction peaks in Fig. 8 are assigned, respectively, to the crystalline planes (111), (200), (220), (311), and (222) of the gold and silver fcc structure by JCPDS:4-0783 and 4-0784 [97].

X-ray Diffraction AuNPs (pink), AgNPs (brown), and Au@AgNPs (red)

Antimicrobial Activity

Monometallic (AgNPs, AuNPs) and bimetallic (Au@AgNPs) materials were tested at four different concentrations:1, 10, 50, and 100 μg/mL. Selected microorganisms to evaluate antimicrobial activity were yeast Candida albicans , Gram-positive bacteria S.aureus , and Gram-negative bacteria E. koli . Growth kinetics curves in a time-lapse of 24 h are shown in Fig. 9.

The growth curves using AuNPs, AgNPs, and Au@AgNPs like inhibitory treatments on Candida albicans (a –c ), Escherichia coli (d –f ), and Staphylococcus aureus (g –i )

Candida albicans

AuNPs show no effect on growth kinetics until 10 h (Fig. 9a), varying in a dose-dependent manner the absorbance reached at 24 h. Interestingly, with 50 µg/mL or more, the growth kinetic shows a steep negative slope from 10 h until reaching a 45% reduction at 24 h, which suggests an antifungal effect of these materials. This can be attributed to the ability of gold nanoparticles to interact with relevant proteins present in fungus such as H ⁺ -ATPase, affecting proton pump activity. This atrophying the ability of yeast to incorporate nutrients causing its death [61]. In Fig. 9b and c was observed that AgNPs and Au@AgNPs inhibit the growth of the yeast Candida albicans from 10 µg/mL. The determination of the MIC₅₀ concentration for both materials was estimated from the dose–response curve shown in Additional file 1:Figure S8. MIC₅₀ is defined as the concentration of nanoparticles that produces a 50% decrease in absorbance concerning the control (yeast without treatment). For AgNPs and Au@AgNPs, MIC₅₀ were 2.21 µg/mL and 2.37 µg/mL, respectively.

However, according to the EDS results (Fig. 9b), the silver content in Au@AgNPs is 64.85% mass. Thus, the concentration of silver in Au@AgNPs for MIC₅₀ is 1.53 µg/mL, 30% lower than in the case of AgNPs. Padmos et. Al. have demonstrated that silver nanoparticles possess important antibacterial features, but are cytotoxic by mammalian cells this effect has reduced using bimetallic nanoparticles especially using gold in the core of bimetallic nanoparticles [35].

Escherichia coli

In Fig. 9d, AuNPs do not show significant inhibition (< 15%) or affect the growth kinetics of E. koli . For AgNPs (Fig. 9e) at low concentrations, the Lag phase remains unchanged, but there is a marked decrease in growth ratio indicated by the slope decrement. At 50 µg/mL Lag phase lasts up to 16 h and viability reaches a maximum of 20% at 24 h. For 100 µg/mL, an apparent detachment of the growth phase of the microorganism is not observed. In Au@AgNPs (Fig. 9f), the first two concentrations do not show changes in their growth phase, but a phase delay of up to 2 h is observed compared to the control. It is interesting to note that the lag phase lasts up to 21 h for the 50 µg/mL concentration, finally there is no explicit growth behavior for the 100 µg/mL concentration.

Staphylococcus aureus

A comparative analysis of lag phase regrowth occurred after 12 h for Au (Fig. 9g), Ag (Fig. 9h), and Au@AgNPs (Fig. 9i) in the case of S. aureus at 50 µg/mL. For the highest concentration at 100 µg/mL, there is no growth of the bacteria. Additionally, we observe changes in the slope of respect control for Au, Ag, and Au@AgNPs at 1 and 10 µg/mL.

AuNPs interaction with these Gram-positive bacteria could be due to the charged surface that causes an electrostatic interaction, destabilizing membrane structure. Similar results, but with higher NPs concentrations, are reported for AuNPs synthesized using Ananas comosus fruit extract as reducing agent [98] and blue-green alga Spirulina platensis protein [99]. Yang dkk. show MIC > 500 µg/mL for S. aureus (CMCC(B)26003), our AuNPs has shown inhibition with 10 times less concentration; in this case, a critical synergy exists with polyphenols molecules on coating and stabilizing the surface of nanoparticle [100]. ROS is generated of less to higher intensity [101] by AuNPs, polyphenols (plant extracts), and AgNPs, so AuNPs in synergy with resveratrol and epigallocatechin gallate (EGCG) promote antibacterial response over S. aureus [102] had the most feasible mechanism in this case. Penders et al. reported 250 and 500 µg/mL of AuNPs-like antibacterial agents over S. aureus increases in bacterial growth lag time and antibacterial effect [61, 98, 99].

We believe that inhibition is caused by AgNPs [103] accumulation and diffusion on bacteria related to NPs surface charges that promote electrostatic interactions [104] with the bacteria's membrane leading to higher penetration and damage. We think this is a similar mechanism described for interactions between E. koli biofilms and AgNps [105].

For Au@AgNPs, the obtained results are comparable to those reported by other workgroups [100, 101]; however, different authors suggest that the inhibition of the growth of the microorganisms is directly related to the thickness of the shell [100, 101]. Core–shell NPs showed low cytotoxicity when tested in NIH-3T3 fibroblasts cells (normal mammalian cells) [35]. A lower proportion of silver in the shell of the Au@AgNPs shows similar results to AgNPs [100], and Au core potentializes antibacterial effect, and minimizes the cytotoxicity.

Curves Growth Analyzed by the Modified Gompertz Model

To know how the growth ratios (µ ) and Lag phase (λ ) are quantitatively modified, the growth curves of microorganisms exposed to different concentrations of nanomaterials (Fig. 10) were adjusted by the Gompertz model (Eq. 2).

Kinetic parameters were obtained by the Gompertz model. Growth Rate µ and Time Lag Phase \(\lambda\) for S. aureus (a , b ) and E. koli (c , d )

In Fig. 10a, it can be seen that all nanomaterials produce a decrease in the replication rate of S. aureus populations when the concentration of nanoparticles increases. This effect results in slightly higher sensitivity for AuNPs. At a concentration of 50 µg/mL, the growth ratio is only 30% concerning control (Additional file 1:Tables S1-S24); at a concentration of 100 µg/mL, all materials inhibit the growth of the S. aureus populasi. The behavior of the adaptive phase for S. aureus with the different treatments is shown in Fig. 10b. It is observed that there are no significant differences in the material used, and at 50 µg/mL, the Lag phase has increased by almost 5 times compared to the adaptive phase of S. aureus (Additional file 1:Tables S1-S24). In general, we can establish that the different nanomaterials evaluated in S. aureus reduce the replication rate and postpone the adaptive phase in a dose-dependent manner until its inhibition at 100 µg/mL.

Figure 10c clearly shows that AuNPs do not affect the growth ratio µ of E. koli bakteri. Meanwhile, AgNPs produce a decrease over µ , reaching a minimum value corresponding to 19% to the control (µ for E.coli without treatment) for 50 µg/mL (see Additional file 1:Table S52). In contrast, Au@AgNPs completely inhibit the E. koli growth at 100 µg/mL. Analysis of the behavior of E. koli Lag phase exposed to different materials is shown in Fig. 10d. In this case, unlike Fig. 10b, each material has a characteristic response. Thus, AuNPs do not generate any modification in the adaptive phase of E. koli , while AgNPs and Au@AgNPs have a dose-dependent effect on the Lag phase, the latter material standing out. Thus, we can establish that AuNPs have no appreciable effect on E. koli bacteria, and Au@AgNPs can inhibit replication and, therefore, indefinitely postpone the Lag phase of E. koli . Interestingly, this effect is not achieved for AgNPs even though the net silver content is higher than in Au@AgNPs. This suggests that the core@shell presentation of both metals produces a synergy that favors antimicrobial activity. Feng et. Al. have reported an electron compensation phenomena from Au to Ag in core–shell and alloy structures, which derive in enhance the cytotoxicity of nanoparticles but kept it the antibacterial properties, that means, a synergy between Au and Ag are assumed, due to observed differences between the monometallic and bimetallic materials [106], but more research is necessary.

Figure 11 shows the results of the direct count study of colonies of microorganisms exposed to nanomaterials. In general, the behavior of the microorganism populations reproduces the results obtained from the growth kinetics study (Fig. 7a, e, i). For example, in Fig. 11a, the population of microorganisms (represented logarithmically) decreases significantly only at 19 h where AuNPs have killed 92% of C. albicans . Interestingly, for nanoparticles containing silver (Fig. 11b, c) a more pronounced population decline is observed. Au@AgNPs system at 50 µg/mL, almost entirely inhibits S. aureus at 7 h (99.5% of bacteria killed) although microorganism reactivates its growth for later times. MBC determination was not possible to obtain for tested concentrations (Additional file 1:Figure S9-11), according to experimental observation higher concentrations are required to show this effect. However, for C. albicans , AgNPs showed an MBC value of 50 µg/mL (Additional file 1:Table S55).

Effect of nanoparticles over microorganisms growth C. albicans exposed to AuNPs (a ), E. koli exposed to AgNPs (b ), and S. aureus exposed to Au@AgNPs (c ). All microorganisms were exposed at 50 µg/mL nanoparticles concentration