Au@Ag Core@Shell nanodeeltjes gesynthetiseerd met Rumex hymenosepalus als antimicrobieel middel

Resultaten en discussies

Karakterisering

Figuur 1a toont de UV-vis-absorptiespectra van de gesynthetiseerde nanomaterialen. De absorpties zijn genormaliseerd voor de maximale gelokaliseerde oppervlakteplasmonresonantie (LSPR) die overeenkomt met elk nanodeeltjessysteem.

Nanodeeltjes UV–Vis-spectra (a ), Z-Potentieel (b ), DLS (c ), en PDI (d ) van Au, Ag en Au@AgNP's

Au@AgNps-absorptiespectrum in figuur 1a heeft een enkele band gecentreerd op 474 nm, die zich tussen de AgNP's LSPR (445 nm) en de AuNP's LSPR (544 nm) bevindt. De afwezigheid van een goudachtige absorptiepiek op Au@AgNP's suggereert dat verkregen nanomaterialen door sequentiële synthese core@shell-structuren zijn. Het is niet mogelijk om een ​​absorptieband te detecteren die geassocieerd is met Au behoort tot de kern. Sommige auteurs nemen aan dat voor core@shell-systemen de absorptiespectra zijn samengesteld uit twee banden die bij elk van de metalen horen voor schaaldiktes tussen 3 en 4 nm. De absorptie geassocieerd met metalcore verdwijnt voor hogere diktes, waardoor een enkele absorptieband wordt verkregen waar de maximale locatie afhangt van de dikte/kerngrootteverhouding van het bimetaaldeeltje [79, 80].

Samal et al. [81] heeft core@shell-nanodeeltjes (Au@Ag) gesynthetiseerd door de kerngroottes te regelen en schillen met verschillende diktes toe te voegen. In het bijzonder valt ons UV-vis-resultaat voor Au@AgNP's samen met dat gerapporteerd door Samal et al. voor 32 nm gouden kernen en een zilverdikte groter dan 15 nm, waarbij spectra worden gekenmerkt door een enkele absorptieband (~  450 nm), en de onderdrukking van Au-oppervlakteplasmonen wordt waargenomen.

Bovendien kan in Fig. 1a absorptie gecentreerd op 280 nm (gebied gemarkeerd in blauw) worden waargenomen, overeenkomend met moleculen van de Rumex hymenosepalus extract gebruikt als reductiemiddel in onze synthese van nanodeeltjes. Aanvullend bestand 1:Afbeelding S1 komt overeen met Rumex hymenosepalus absorptiespectrum van waterige oplossingen. Een karakteristieke band gecentreerd op 278 nm wordt waargenomen, geassocieerd met de elektronische overgangen van de aromatische ringen geconjugeerd met de carbonylgroepen van polyfenolische verbindingen [82]. Deze absorptieband in de UV-Vis-spectra van nanodeeltjes geeft aan dat eindproducten extractmoleculen bevatten die erin achterblijven, Rivero-Cruz et. al. hebben vier stilbenoïden, twee flavan-3-olen en drie antrachinonen geïsoleerd uit R. hymenosepalus [83] en Rodríguez-León et. al. hebben een onderzoek naar nucleaire magnetische resonantie uitgevoerd om vast te stellen dat Rumex hymenosepalus bevatten belangrijke moleculen als stilbeenglycoside en epicatechinegallaat en epigallocatechinegallaat [61]. Deze moleculen nemen als reductiemiddelen deel aan de synthese van nanodeeltjes. Het proces omvat de deprotonering van enkele -OH-groepen van fenolische ringen om  = CO-groepen te vormen. Polyfenolische moleculen worden geoxideerd en de vrijgekomen elektronen worden overgebracht naar Ag ⁺ en Ag ³⁺ ionen om Au ⁰ . te vormen en Ag ⁰ .

Figuur 1b toont de Zeta-potentialen die overeenkomen met AuNP's, AgNP's en Au@AgNP's bij concentraties van 1, 10, 50 en 100 µg/ml. Nanodeeltjes worden gedispergeerd in ultrapuur water en de meting werd in drievoud bij 25 ℃ uitgevoerd. Over het algemeen vertonen nanodeeltjes over het hele concentratiebereik zeta-potentiaalwaarden die negatiever zijn dan -30 mV, en bereiken ze waarden van -40 mV bij een concentratie van 100 µg/ml voor AuNP's en Au@AgNP's en -38 mV voor AgNP's. Deze zeer negatieve Zeta-potentiaalwaarden geven aan dat nanodeeltjes afstotende interacties tussen hen ervaren die hun aggregatie voorkomen en de stabiliteit van metaalcolloïden op lange termijn mogelijk maken [84,85,86]. Door de UV-Vis-spectroscopieresultaten te correleren met de verkregen Zeta-potentiaalwaarden, kunnen we vaststellen dat de zeer negatieve waarden te wijten kunnen zijn aan de complexvorming van polyfenolische moleculen van het extract op het oppervlak van de nanodeeltjes [87].

Voor biologische toepassingen is het waardevol om een ​​populatie van nanodeeltjes met monodisperse afmetingen [88] te verkrijgen, dus Fig. 1c en d tonen de DLS-waarden die overeenkomen met de gemiddelde diameter en polydispersiteitsindex (PDI) voor AuNP's, AgNP's en Au@AgNP's bij concentraties van 1, 10, 50 en 100 µg/ml. De gemiddelde diameter van Au@AgNP's is ongeveer 250 nm en wordt constant gehouden als een functie van de concentratie, op een vergelijkbare manier voor monometallische nanodeeltjes met een gemiddelde diameter van respectievelijk 122 en 135 nm voor AuNP's en AgNP's. Merk in hetzelfde geval op dat de PDI-waarden rond de 0,3 liggen voor monometallische nanodeeltjes en 0,2 voor Au@AgNP's. Deze resultaten, waarbij de grootte niet varieert met de concentratie, geven aan dat de verschillende nanodeeltjessystemen een goede stabiliteit hebben met een matige polydispersiteit van groottes (0,3 ≥ PDI ≥ 0,2).

De optische band gap van Tauc-plot werd berekend (aanvullend bestand 1:figuur S2). Geleidingsband door halfgeleidermaterialen hebben waarden E _g < 3 eV, als referentie heeft germanium E _g < 0.7 eV en silicium is E _g = 1.1 eV [89]. In ons geval is de band gap voor Au@AgNP's, AuNP's en AgNP's respectievelijk 1,93 eV, 2,03 eV en 2,33 eV. Dit betekent dat deze materialen worden beschouwd als halfgeleiders, en deze functie is verkregen voor de kwantumopsluitingseffecten die een grotere energiekloof veroorzaken, zodat nanomaterialen kunnen worden gebruikt als sensoren, batterijen en opto-elektronische apparaten [64].

Om vast te stellen of organische verbindingen aanwezig zijn in nanodeeltjes, werden deze nanomaterialen gekarakteriseerd door XPS en FTIR. Figuur 2a toont de onderzoeksspectra van het plantenextract en de nanodeeltjes. Zoals te zien is, presenteren alle nanodeeltjessystemen de karakteristieke signalen die horen bij de elementen koolstof (C 1 s, 284,6 eV) en zuurstof (O 1 s, 532,2 eV) van de Rumex hymenosepalus extract. Dit resultaat bevestigt dat organische moleculen uit extract aanwezig zijn in verkregen nanomaterialen. Bovendien werden XPS-spectra met hoge resolutie verkregen om de oxidatietoestanden van zilver en goud in de nanodeeltjes te analyseren (aanvullend bestand 1:figuur S3). Voor AgNP's (aanvullend bestand 1:figuur S3A) kunnen de signalen bij bindingsenergieën van 373,76 eV en 367,76 eV (∆BE = 6,0 eV) overeenkomend met de 3d3/2 en 3d5/2 spectraallijn worden geassocieerd met Ag ⁰ (Metallisch zilver). Voor AuNP's (aanvullend bestand 1:figuur S3B) bevindt de piek geassocieerd met 4f5/2 spin-orbitale koppeling zich op bindingsenergie 87,65 eV. Voor 4f7/2 bevindt de piek van de spin-orbitale koppeling zich op 83,98 eV. De verhouding van intensiteiten (I4f7/2 > I4f5/2), locatie en scheiding tussen pieken (ΔBE = 3.67 eV) bevestigen dat goudionen (Au ³⁺ ) worden volledig gereduceerd tot metallic goud Au ⁰ [90]. Voor Au@AgNP's worden XPS-spectra met hoge resolutie die overeenkomen met zilver en goud weergegeven in respectievelijk aanvullend bestand 1:figuur S3C en figuur S3-D. Deze spectra vertonen hetzelfde gedrag als hun monometallische tegenhangers. Dit geeft aan dat zowel zilver als goud nulwaardig zijn in de core@shell-presentatie.

XPS-onderzoeksspectra (a ), en FTIR (b ) voor Rh, Ag, Au en Au@AgNP's

FTIR in Afb. 2e toont op 3296 cm ⁻¹ hydroxylgroepen, 2974 cm ⁻¹ (C–H) binding van de aromatische ring, 1689 cm ⁻¹ geassocieerd met uitrekking van carbonylgroepen (C=O) en 1689–1400 cm ⁻¹ vanwege carboxylaatbinding (C–O) en strekkende koolstof-koolstofbinding en 1212 cm ⁻¹ geassocieerd met fenol C–O stretching geconjugeerd met AuNP's, 1049 cm ⁻¹ C–O-rekmodus van de catechine-estergroepmoleculen [88,89,90] en 799 cm ⁻¹ de buiging uit het vlak in het fenol, C–H-buiging gerapporteerd bij 964,4 en 829,39 cm ⁻¹ kenmerk van resveratrol [91] verschoven in Rh-extract naar 1031 en 867 cm ⁻¹ , en AuNP's, AgNP's en Au@AgNP's zijn ook verschoven, wat indicatief is voor de complexering van moleculen polyfenolen van Rh-extract met nanodeeltjes.

Figuur 3a komt overeen met een representatieve helderveld-STEM-microfoto van het Au@AgNPs-systeem bij lage vergroting (schaalbalk van 100 nm). Een reeks nanodeeltjes zonder agglomeratie en met voornamelijk quasi-sferische geometrie is te zien. 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 ) en (b ), AgNPs in (c ) en (d ), and Au@AgNPs in (e ) en (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. coli . 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 (een –c ), Escherichia coli (d –f ), and Staphylococcus aureus (g –ik )

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. coli . 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 et al. 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. coli 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 (een , b ) and E. coli (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 population. 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. coli bacteriën. 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. coli growth at 100 µg/mL. Analysis of the behavior of E. coli 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. coli , 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. coli bacteria, and Au@AgNPs can inhibit replication and, therefore, indefinitely postpone the Lag phase of E. coli . 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. coli exposed to AgNPs (b ), and S. aureus exposed to Au@AgNPs (c ). All microorganisms were exposed at 50 µg/mL nanoparticles concentration