Electrochemically Deposited SolâGel Based Nanoparticle-Imprinted

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Electrochemically Deposited Sol-Gel Based Nanoparticles Imprinted Matrices for the Size-Selective Detection of Gold Nanoparticles Netta Bruchiel-Spanier, Gianmarco Giordano, Atzmon Vakahi, Massimo Guglielmi, and Daniel Mandler ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01215 • Publication Date (Web): 07 Sep 2018 Downloaded from http://pubs.acs.org on September 10, 2018

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ACS Applied Nano Materials

Electrochemically Deposited Sol-Gel Based Nanoparticles Imprinted Matrices for the SizeSelective Detection of Gold Nanoparticles Netta Bruchiel-Spanier1, Gianmarco Giordano2, Atzmon Vakahi1, Massimo Guglielmi2, Daniel Mandler1* 1

2

Institute of Chemistry, the Hebrew University of Jerusalem, Jerusalem 9190401, Israel

Dipartimento di IngegneriaIndustriale, Università di Padova, Via Marzolo 9, 35135 Padova, Italy

KEYWORDS sol-gel, nanoparticles, imprinting, electrochemistry, Focused Ion Beam

ABSTRACT

Nanoparticle imprinted matrices (NAIMs) is a new approach, in which nanoparticles (NPs) are imprinted in a matrix followed by their removal to form highly selective voids that can recognize the original NPs. In this study, the effect of a sol-gel matrix on the imprinting and reuptake of gold nanoparticles (AuNPs) is examined. Specifically, indium tin oxide (ITO) films were modified with a positively charged polymer on which the negatively charged AuNPs stabilized with citrate (AuNPs-cit) were adsorbed. This was followed by the electrochemical deposition of

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sol-gel matrices with different thickness and functional groups onto the ITO/AuNPs-cit. Electrochemical oxidation dissolved the AuNPs-cit and formed cavities in the sol-gel films, which fit both the size and shape of the AuNPs-cit. Reuptake of these NPs from an aqueous solution was successful using the imprinted films, whereas the non-imprinted films did not reuptake the AuNPs-cit. Furthermore, the thickness of the sol-gel layers as well as the type of the silanes that were deposited, play an important role on the recognition ability of the NAIM. Finally, we found that the NAIMs are selective and larger AuNPs-cit were not recognized by the imprinted matrix.

Introduction: Particles with nanoscale dimensions present extraordinary physical and chemical properties that do not exist in the macroscopic scale1. The potential use of nanomaterials in fields such as medicine2-4, sensing5-7, catalysis8, is constantly growing and is expecting to have significant impact on human health and the environment9. Yet, numerous studies show that nanoparticles (NPs) are toxic to biological systems because of their ability to penetrate different membranes and accumulate in the cells10-14. Additionally, the environmental impact of NPs in the future is unclear due to the lack of regulation and statistical knowledge in this field15. At the same time, the applications of NPs in various areas including medicine will continue to attract further research and development in spite of the current lack of knowledge about their toxicity. A notable example is the use of gold16 or iron oxide NPs17 in medical imaging by CT and MRI. It is fairly well documented that nanotoxicology is associated not only with the material that constitutes the NPs; but also, with their size, shape, and in particular, the stabilizing shell. The


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latter governs the interfacial properties of the NPs, such as the permeation and accumulation across and in biological tissues. We have recently introduced the term "nanoparticle speciation", which refers to the differentiation of NPs based on their size, shape and shell18, similarly to the classification used for heavy metals. Hence, there is an unmet need for analytical tools for NPs speciation. The nowadays available techniques for NPs detection, such as dynamic light scattering19, coulter counters20, electrochemical detection by nano-impact21-22, electron microscopy, and centrifuge particle size analysis23 do not provide full speciation ability. Namely, these analytical methods provide information only about the size and concentration of the NPs; however, they do not distinguish between NPs having different shapes and stabilizing shells. Nanoparticle imprinted matrices (NAIMs) is a new approach introduced recently by us18, 24-26, in which NPs are imprinted in a matrix followed by their removal to form highly selective voids that can recognize the original NPs. The NAIM method is derived from the well-established molecularly imprinted polymer (MIP) approach27-29. NAIMs involve the preparation of a thin matrix in which NPs are embedded, followed by their release. The matrix imprinted with voids is subsequently used for the selective reuptake of the NPs. Recently, we showed several different approaches for recognizing NPs based on their size18, 24-26. Our first approach took advantage of the Langmuir-Blodgett (LB) method, which was used for co-depositing a matrix made of polyaniline or cellulose acetate and AuNPs18, 24. These systems exhibited selectivity towards the size of the NPs, which was varied by either the metallic core or the stabilizing shell. A second approach was demonstrated by Witt et al. who formed an insulating matrix around Au and Ag NPs by electropolymerization of phenol26. They showed size selectivity towards different core NPs materials with the same stabilizing shell.


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The interactions between the surrounding matrix and NPs influence significantly onto the recognition ability of the system. Therefore, an essential step for the NAIM preparation must include seeking for the optimal matrix, which best interacts with the NPs. Here, we present a different approach that is based on sol-gel chemistry. Forming the NAIM of sol-gel precursors provides unlimited flexibility in designing the matrix in which the NPs are imprinted. Sol-gel thin films are typically deposited from solution by spin coating30-31, dip coating32-34 and spraying35. We have developed a sol-gel deposition process driven by electrochemistry36-39. This is based on accelerating the hydrolysis and condensation of the sol-gel precursors by applying a negative potential, which increases the pH on the electrode surface39. A major advantage that the electro-assisted sol-gel deposition approach offers is the extremely fine control of the thickness of the deposited layer, vide-infra. In this study (Figure 1) citrate stabilized AuNPs (AuNPs-cit) were adsorbed onto a poly(diallyldimethylammonium chloride) (PDDA) modified indium tin oxide (ITO) surface. This was followed by the electrochemical deposition of sol-gel matrices made of different precursors. The thickness of the sol-gel thin layers was very carefully controlled in the course of the electrodeposition process. The AuNPs-cit were removed by electrochemical oxidation and the ability of the imprinted sol-gel matrix to recognize the AuNPs-cit selectively was evaluated. We found that the thickness of the sol-gel layers and the type of silanes that were deposited play a crucial role on the recognition of the NAIM. Furthermore, the NAIM showed size-selective recognition of AuNPs-cit. Figure 1 Experimental Section: Materials:


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HAuCl4.3H2O, tetraethyl

orthosilicate (TEOS),

3-aminopropyltriethoxysilane

(APTES),

potassium hexacyanoferrate(III), ferrocenemethanol, hydrogen chloride acid 37% (reagent grade), nitric acid 70% (reagent grade), poly(diallyldimethylammonium chloride) (20 wt. %, PDDA) were purchased from Sigma-Aldrich. Ethanol (reagent grade) was ordered from J. T. Baker. Acetone (AR grade) and potassium chloride (AR grade) were obtained from Gadot. Potassium nitrate (analysis grade) was purchased from Merck. All chemicals were used as received. One-side coated indium tin oxide (ITO) plates were purchased from Delta Technologies (CG-601N-CUV, Stillwater, MN, US). Ultrapure deionized water (Easy Pure UV, Barnstead) was used for all aqueous solutions. Instruments: Chronoamperometry, linear sweep voltammetry (LSV) and cyclic voltammetry (CV) were conducted

with

a

potentiostat

(CHI-630,

CH

Instruments

Inc.)

using

a

three

electrode glass cell. An Ag/AgCl (1 M KCl) and graphite rod (99.9995%) were used as reference and counter electrodes, respectively. Extra high resolution scanning electron microscopy (XHRSEM, Magellan XHR 400L, FEI) was used. High-resolution transmission electron microscopy (HR-TEM) (Tecni F20 G2) and Focused Ion Beam (460F1 Dual Beam, FEI Helios Nano Lab) were employed to characterize the NAIMs. Methods: Synthesis of gold nanoparticles (AuNPs): 8 nm Gold nanoparticles stabilized with citrate (AuNPs-cit) were synthesized based on the Turkevich40 procedure with some minor changes. Specifically, 97 mg of sodium citrate was dissolved in 150 mL of water (2.2 mM) and heated until boiling under vigorous stirring. Then, 1 mL of an aqueous solution of 25 mM HAuCl4.3H2O was added and stirred for 10 minutes until red color was obtained. The resulting solution was


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allowed to cool and diluted from 150 to 450 mL. This was the only stock solution of AuNPs-cit that was used. For selectivity tests, 30 nm AuNPs-cit were prepared using the 8 nm AuNPs-cit as seeds. Based on Bastus,41 after the formation of the 8 nm AuNPs-cit, the mixture was cooled down to 90 oC and 1 mL of 25 mM HAuCl4.3H2O was added. The solution was stirred for 30 min followed by injection of additional 1 mL of 25 mM HAuCl4.3H2O. Then, the mixture was stirred for 30 min and diluted by removing 55 mL of the solution and adding 2 mL of 60 mM sodium citrate and 53 mL of distilled water. This solution was used as seeds and the process was repeated twice more. Preparation of the electrodeposition solution: 10 mL of 0.1 M KNO3 solution were mixed with 10 mL of ethanol. Into this solution, either 100 mM of TEOS or 50 mM of TEOS and 50 mM of APTES were added. Into the two solutions 0.1 M HCl was added dropwise until pH 3 was obtained. The solutions were stirred for additional 1 h at room temperature to hydrolyze the silanes. Sample preparation: ITO surfaces were cleaned by sonication in acetone, ethanol, and deionized water for 10 minutes each. Then, the ITO films were immersed into a 0.01 wt% PDDA solution for 20 minutes with mild shaking. The plates were washed three times with water and placed vertically in a solution of AuNPs-cit for 1.5 h. After careful washing, the PDDA modified ITO samples were immersed into the desired silane solution followed by applying constant negative potential (−1 V vs. Ag/AgCl, (1 M KCl)) for different times. The samples were gently washed with water and left to dry overnight. Removal and reuptake of the AuNPs-cit: the removal of the imprinted AuNPs-cit from the matrix was achieved by electro-oxidation using three electrodes cell in 0.1 M KCl solution. Several linear sweep voltammetry (LSV) scans at 0.1 Vs-1 from 0 to 1.3 V were performed until


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no oxidation peak was observed, indicating the complete removal of the AuNPs-cit. the reuptake of the AuNPs-cit was performed by immersing the samples into a solution containing the AuNPs-cit for 1.5 h followed by careful washing with water. The reuptake by the NAIM was evaluated by LSV. Lamella Preparation for TEM imaging: The samples were first imaged using SEM and an area of ca. 8x2 µm2 was chosen. Then, this area was coated with a 150 nm thick Pt protecting layer using electron beam. This was carried out in order to protect the surface from the gallium ions, which were used to cut a 5 µm deep lamella. The obtained lamellas were separated from the samples and connected to a grid. Finally, the lamellas were thinned to a width of ca. 50 nm and inserted to the TEM. Results and discussion: The essence of our approach (Figure 1) comprises the adsorption of AuNPs-cit on a conducting surface, followed by electrodeposition of the sol-gel to form the NAIM. Adsorbing the negatively charged AuNPs-cit was facilitated by preadsorbing PDDA, a positively charged polymer. Figure S1 shows SEM image of ITO film after embedding AuNPs-cit. It can be seen that the AuNPs-cit distribution is uniform onto the ITO film. Two different sol-gel matrices were formed for studying the effect of the matrix on the recognition ability of the system. One contained only TEOS (termed Au-TEOS) while the other contained a mixture of TEOS and APTES (termed Au-TEOS/APTES). The sol-gel matrices were electrodeposited by applying a constant negative potential, which generated hydroxyl ions on the electrode and therefore accelerated the condensation of the silanes36, 39. Based on our previous work, the thickness of the electrodeposited silane films depends primarily on the applied potential as well as deposition time. Hence, we applied a constant potential and varied the time. Specifically, we deposited the


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sol-gel for 30, 60 and 120 s at −1 V vs. Ag/AgCl (1 M KCl). Blank experiments were conducted identically, yet, without the AuNPs-cit. Figure 2 shows the CV of ITO before and after the electrodeposition of TEOS for both AuTEOS (Figure 2A) and blank (Figure 2B) samples in a solution of Fe(CN)63- and 0.1 M of KCl. It can be seen, that in both cases, as the electrodeposition time increases, the oxidation and reduction currents decrease. This is presumably due to increasing the thickness of the insulating sol-gel layer. Furthermore, the oxidation and reduction peak potentials shift to more positive and negative potentials, respectively, for both cases (Figure 2A-B) indicating slower kinetics of the electrochemical process with increasing deposition time. However, the decrease in Au-TEOS (Figure 2A) was more moderate than films without preadsorbed AuNPs-cit (Figure 2B), probably due to the electrical conductivity of the AuNPs-cit, which allows electron transfer of the iron complex. This also implies that the TEOS was polymerized around the AuNPs-cit, leaving them partially exposed to the solution. We have shown in a previous study42 that the change in pH, which occurs at the electrode-electrolyte interface is not local and spreads over a distance of several tens of nanometers. Indeed, the reduction of either oxygen or water should be more facile on Au than on ITO, yet, the AuNPs are covered with citrate and do not have hydroxyl residues to which the sol-gel film can be covalently attached. Hence, it might be that a sol-gel film grows also on the AuNPs; however, it will be removed upon washing the sample.

Figure 2 We would like to note that based on our experience it is very difficult to remove the embedded AuNPs-cit from the matrix and therefore, there is no spontaneous loss of the AuNPs-cit, which


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might leave holes in the matrix. Therefore, all the changes in the currents shown in Figure 2A are only due to the presence of pre-adsorbed AuNPs-cit. To better characterize the sol-gel matrix we imaged the cross-section of the films using FIB, energy filtered TEM (EFTEM) and TEM. We expected that the thickness of the matrices would be of the order of the NPs, which posed a technical challenge in imaging the embedded NPs. Figure 3 shows TEM and EELS analysis of the cross-section of a lamella cut from an Au-TEOS sample, where the sol-gel was deposited for 120 s. The elemental map of O and Si are shown in Figure 3B-C. The cross-section shows the ITO on glass on which the thin Au-TEOS was deposited. From the elemental maps of O and Si it can be seen that the Au-TEOS matrix contains both O and Si, which indicates that the matrix is indeed a silica based layer. Electrochemistry provides statistical information on the film and is highly sensitive to surface defects, e.g. pinholes, while TEM characterizes the film locally. Each sample was analyzed using electrochemistry (Figure 2) and its cross section was imaged in several random areas using TEM (Figures 3 and 4). In all cases, a homogenous layer was obtained and no defects were detected either by electrochemistry or by TEM. Based on these results, we conclude that the sol-gel layer is highly homogeneous and continuous.

Figure 3 Figure 4 shows TEM images of lamellas cut from Au-TEOS samples deposited for 30, 60 and 120 s. Embedded AuNPs-cit can be clearly seen inside the sol-gel layer in each sample. As the deposition time increased from 30 to 60 and 120 s, the thickness of the layer increased from 3 to 5 and 7.5 nm, respectively. This demonstrates nicely the control of the thickness of the electrodeposited films at the nanometer scale using electrochemistry. The visibility of the


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AuNPs-cit is due to their stronger interaction with the TEM beam. The size of the embedded AuNPs-cit is ca. 8 nm. Comparing the AuNPs-cit size to the thickness of the sol-gel layer shows that increasing the electrochemical deposition time results in higher coverage of the AuNPs-cit by the matrix. Specifically, the AuNPs-cit embedded in the sol-gel that was deposited for 30 s are well exposed above the matrix (Figure 4A) whereas those embedded in the matrix that was deposited for 120 s are almost fully covered by it (Figure 4C). These results are in agreement with the CV measurements, which indicate that the rate of electron transfer decreases with increasing the deposition time. The thickness of a layer deposited electrochemically for 60 s (Figure 4B), is approximately half the size of the embedded AuNPs-cit. We will show that this is an ideal thickness, which forms the optimal cavities that are essential for recognizing the AuNPscit. Figure 4 So far, we have used the simplest monomer, i.e., TEOS, as the building block for the matrix. Since the interactions between the AuNPs-cit and the matrix are likely to play a major role in the reuptake capability of the matrix, it is logical to employ other functionalized silanes that could intensify these interactions. Among the large arsenal of commercially available silanes, we have chosen to test 3-aminopropyltriethoxysilane (APTES) because of its potential to form electrostatic as well as hydrogen bonds with the citrate stabilized AuNPs. Accordingly, we formed Au-TEOS/APTES matrices following the same procedure as discussed above; however, using a deposition solution consisting of 1:1 (molar ratio) of TEOS and APTES. Figure 5 shows the CV of ITO before and after a matrix of TEOS/APTES was electrodeposited for various times (under the same potential) with and without pre-adsorbing of AuNPs-cit. The CVs were carried out in a solution of 0.5 mM ferrocenemethanol and 0.1 M of


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KCl. Ferrocenemethanol was used to eliminate any effects of electrostatic interactions between the matrix and the redox couple. As is evident, the electrochemical deposition of the silanes causes a significant decrease in the oxidation and reduction currents and reversibility of the ferrocenemethanol. The small but clear changes of the CV in both cases, namely, in the absence and presence of preadsorbed AuNPs-cit, indicate that a thicker sol-gel layer is built with increasing the time of deposition. Yet, while the changes in the current and reversibility, i.e., potential difference between the anodic and cathodic peaks, are gradual with the deposition time in the presence of preadsrobed AuNPs-cit, the observed changes of the CV in the absence of preadsorbed AuNPs-cit, are minute. Increasing the potential difference between the peaks in the absence of AuNP-cit indicates that the sol-gel layer is built with increasing the deposition time. The decrease of the current is more pronounced in the presence of the NPs (Figure 5A) suggesting that a thicker film is built on the ITO where AuNPs-cit were adsorbed prior to sol-gel electrodeposition. This is conceivable taking into account the faster kinetics of water reduction on Au than on ITO39, which is responsible for elevating the pH on the electrode surface and the driving force for solgel deposition. It should be noticed that the CV of ferrocenemethanol recorded with AuTEOS/APTES shows somewhat an opposite behavior (as compared with Fe(CN)63- for AuTEOS) namely, that the electrode is blocked more rapidly when the film contains AuNPs-cit. We attribute the faster film formation on the mixed TEOS/APTES layer to the attraction of the positively charged monomer, APTES, by the negatively charged AuNPs-cit. This probably leads to the denser coverage of the AuNPs-cit by the TEOS/APTES matrix, which inhibits electron transfer across the layer. Figure 5


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The next step after forming the NAIMs comprises the removal of the AuNPs-cit by electrochemical oxidation. Figure 6 represents the LSV of the ITO surfaces coated with silanes for different deposition times. It is worth mentioning that the amount of AuNPs-cit adsorbed in each sample is similar. The peak at ca. 0.84 V vs. Ag/AgCl is associated with the oxidation of the AuNPs-cit to form soluble AuCl4−. Integration of the oxidation peak yields the charge from which the amount of AuNPs-cit can be calculated providing that their diameter is known. It can be seen that the charge (and therefore the amount of AuNPs-cit) decreases as the deposition time of the TEOS increases (Figure 6A) whereas it is smaller and indifferent to the deposition time for the mixed TEOS/APTES film (Figure 6B). This implies that for the Au-TEOS based NAIM, as the deposition time increases more particles are entrapped inside the matrix and are not exposed to the chloride solution. On the other hand, the observed oxidation peak for the mixed film was similar to the peak obtained for depositing TEOS for 120 s. This is probably due to the faster coverage of the AuNPs-cit with the TEOS/APTES layer, which inhibits their removal and is in accordance with the effect of deposition on the CV shown above (Figure 5A). To further support this hypothesis, we imaged the different films of Au-TEOS and Au-TEOS/APTES after the oxidation process (see Figure S2). It can be seen, that for the TEOS based NAIM, as the deposition time increases the amount of AuNPs-cit remained on the surface also increases (less particles were oxidized) while for the TEOS/APTES base NAIM, the amount is independent on the deposited time and similar to that of TEOS deposited for 120 s. Figure 6 To prepare the films for the recognition test, the oxidation process was repeated until complete removal of all of the exposed AuNPs-cit was achieved. Following, the Au-TEOS, AuTEOS/APTES and the control films were immersed in an aqueous solution containing the


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AuNPs-cit for 1.5 h. The reuptake process was further quantified by electrochemically oxidizing the AuNPs-cit in the NAIMs in 0.1 M KCl solution. Figure 7 shows the LSV of the originally imprinted Au-TEOS NAIMs, the films after reuptaking the AuNPs-cit, and the control test. The latter is a film which was not imprinted by the AuNPs-cit. In a previous paper, we have introduced the term reuptake percentage, which was defined as the percent ratio between the charge of the reuptake oxidation wave and that of the original NAIM18. The reuptake percentage for the different times of sol-gel deposition is summarized in Table 1. Figure 7 It can be seen that the reuptake percentage decreases with increasing the time of the electrochemical deposition of the sol-gel. In spite of the fact that the efficiency for 30 s deposition time seems higher, one must not neglect the control (non-imprinted film) measurement. A significant oxidation wave is observed for the non-imprinted film of 30 s deposition time, indicating recognition by non-specific adsorption. This is understandable taking into account the very thin thickness of the TEOS layer, which is approximately 2 nm (according to the TEM image). Table 1 This thickness allows not only electron transfer from particles that are entrapped in the original cavities, but also from those, which adsorb spontaneously onto the sol-gel surface. Increasing the deposition time substantially to 120 s, results in a 7 nm sol-gel layer (see Figure 4C) which inhibits the ability to reuptake the AuNPs-cit by the cavities that are buried deeply in the matrix. On the other hand, the layer is sufficiently thick to prevent electron transfer from particles that are non-specifically adsorbed on the sol-gel surface. Hence, an optimal layer, with a thickness of ca. 4 nm is achieved (Figure 4B) upon deposition a sol-gel film for 60 s. Such a film is, on one


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hand, thick enough to prevent non-specific recognition, and on the other hand, exhibits efficient reuptake of the AuNPs-cit. This is due to the thickness being approximately of the same size as the radius of the AuNPs-cit, which enables facile capturing by the imprinted cavities. A different behavior is observed, when depositing TEOS/APTES layers as seen in Figure 8. The APTES in the matrix, which is positively charged, attracts the negatively charged AuNPs-cit leading to substantial non-specific adsorption. This explains the large oxidation wave observed for the shortest deposition time of 30 s in the control experiment (Figure 8A). Clearly, the 30 s of deposition does not block electron transfer across the layer. Figure 8 Extending the deposition time suppresses the non-specific recognition and allows only the recognition of the AuNPs-cit, which are reuptaken by the imprinted cavities in the matrix (Figure 8B-C). The reuptake of the TEOS/APTES is worse than that of the TEOS due to the partial coverage of the cavities as a result of the interaction between the embedded nanoparticles and the APTES as discussed above. Finally, we conclude that the matrix thickness is crucial to prevent non-specific recognition and to allow only the recognition of AuNPs that enter the cavities forms after their extraction. From the above discussion, it is clear that the TEOS matrix with deposition time of 60 s represent the best NAIM system due to its high reuptake ability and low non-specific recognition. Therefore, we examined the selectivity of this matrix towards AuNPs-cit differing in their size. After the removal of the 8 nm AuNP-cit, the sample was immersed into a solution of 30 nm AuNPs-cit for 1.5 h. Figure 9 shows the oxidation waves of TEOS based NAIM with deposition time of 60 s after reuptake of 8 nm or 30 nm AuNPs-cit. The difference between the oxidation wave of


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reuptake 8 and 30 nm AuNPs-cit is remarkable, which clearly indicates the size-selectivity of the system. This implies that the obtained cavities preserve their shape, and therefore, only NPs that fit the size of the original AuNPs-cit will be detected. The selectivity of the TEOS bases NAIM was also tested by comparing the reuptake of the AuNPs-cit to AuNPs with the same size; however, stabilized with 3-mercaptopropionic acid (mpa). We found that in this case, the matrix was not selective, and AuNPs-mpa entered the cavities imprinted by the AuNPs-cit. Therefore, it is likely that the nature of the shell alone is insufficient to differentiate between the NPs (that have similar surface charge). We conclude that this NAIM system is sensitive to changes in the size of NPs but less sensitive to the functionalities on the NP surface. Figure 9 Conclusions: A new approach for the formation of nanoparticles imprinted matrices (NAIMs) based on the electrochemical sol-gel deposition is demonstrated. The process involves pre adsorption of Au nanoparticles stabilized by citrate onto indium tin oxide following by electrodeposition of sol-gel on the non-occupied areas. The thickness of the matrix can be controlled at the nanometer scale. This tuning is crucial since a thinner layer leads to non-specific recognition (due to adsorption on top of the sol-gel layer) while a too thick layer does not allow the reuptake of the AuNPs-cit presumably due to partial blocking of the cavities by the sol-gel. We found that the optimal thickness should be similar to the radius of the nanoparticle. For the first time, we have been able, using FIB, to observe the embedded AuNPs-cit inside the matrix, which supports the importance of the layer thickness on the reuptake efficiency. Two silanes were employed for constructing the matrix; tetraethoxysilane and 3aminopropyltriethoxysilane. Clearly, the latter interacts more strongly with the negatively


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charged nanoparticles. Surprisingly, this reduced the efficiency of the reuptake of the nanoparticles because of the accumulation of the sol-gel layer on top of the embedded nanoparticles, which enables their removal (by anodic dissolution); however, blocks their reuptake. Hence, we are currently designing better matrices with others sol-gel precursors that will interact by supramolecular chemistry with the nanoparticles rather than by electrostatic interactions. We believe that by this approach, the affinity of the matrix towards the nanoparticles can be significantly improved. Finally, we found that this NAIM system is sizesensitive were only NPs that fit the size of the original AuNPs-cit were detected while larger AuNPs-cit were not recognize. Further work to construct NAIMs that will be able to differentiate NPs based on their shell is currently undertaken. AUTHOR INFORMATION Corresponding Author * [email protected] Author Contributions All authors have given approval to the final version

ACKNOWLEDGMENT

This research is supported by the Israeli Ministry of Science and Technology (contract 313575). The Harvey M. Krueger Family Center for Nanoscience and Nanotechnology of the Hebrew University is acknowledged.

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ASSOCIATED CONTENT One Supporting Information file. SEM image of AuNPs-cit on ITO-PDDA surface and on AuTEOS and Au-TEOS/APTES surfaces after the oxidation process. References: 1. Alivisatos, A. P., Semiconductor Clusters, Nanocrystals, and Quantum Dots. Science 1996, 271, 933-937. 2. Fernández, T. D.; Pearson, J. R.; Leal, M. P.; Torres, M. J.; Blanca, M.; Mayorga, C.; Le Guével, X., Intracellular accumulation and immunological properties of fluorescent gold nanoclusters in human dendritic cells. Biomaterials 2015, 43, 1-12. 3. Giner-Casares, J. J.; Henriksen-Lacey, M.; Coronado-Puchau, M.; Liz-Marzán, L. M., Inorganic nanoparticles for biomedicine: where materials scientists meet medical research. Materials Today 2016, 19, 19-28. 4. Zhang, Y.; Yan, M.; Wang, S.; Jiang, J.; Gao, P.; Zhang, G.; Dong, C.; Shuang, S., Facile one-pot synthesis of Au(0)@Au(i)-NAC core-shell nanoclusters with orange-yellow luminescence for cancer cell imaging. RSC Advances 2016, 6, 8612-8619. 5. Roy, S.; Palui, G.; Banerjee, A., The as-prepared gold cluster-based fluorescent sensor for the selective detection of AsIII ions in aqueous solution. Nanoscale 2012, 4, 2734-2740. 6. Guan, G.; Zhang, S.-Y.; Cai, Y.; Liu, S.; Bharathi, M. S.; Low, M.; Yu, Y.; Xie, J.; Zheng, Y.; Zhang, Y.-W.; Han, M.-Y., Convenient purification of gold clusters by coprecipitation for improved sensing of hydrogen peroxide, mercury ions and pesticides. Chemical Communications 2014, 50, 5703-5705. 7. Yuan, X.; Luo, Z.; Yu, Y.; Yao, Q.; Xie, J., Luminescent Noble Metal Nanoclusters as an Emerging Optical Probe for Sensor Development. Chemistry – An Asian Journal 2013, 8, 858871. 8. Li, G.; Jin, R., Atomically Precise Gold Nanoclusters as New Model Catalysts. Accounts of Chemical Research 2013, 46, 1749-1758. 9. Jin, R.; Zeng, C.; Zhou, M.; Chen, Y., Atomically Precise Colloidal Metal Nanoclusters and Nanoparticles: Fundamentals and Opportunities. Chemical Reviews 2016, 116, 1034610413. 10. Colvin, V. L., The potential environmental impact of engineered nanomaterials. Nat Biotech 2003, 21, 1166-1170. 11. Behra, R.; Krug, H., Nanoecotoxicology: Nanoparticles at large. Nat Nano 2008, 3, 253254. 12. Lewinski, N.; Colvin, V.; Drezek, R., Cytotoxicity of Nanoparticles. Small 2008, 4, 2649. 13. McCall, M. J., Environmental, health and safety issues: Nanoparticles in the real world. Nat Nano 2011, 6, 613-614.


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14. Sajid, M.; Ilyas, M.; Basheer, C.; Tariq, M.; Daud, M.; Baig, N.; Shehzad, F., Impact of nanoparticles on human and environment: review of toxicity factors, exposures, control strategies, and future prospects. Environmental Science and Pollution Research 2015, 22, 41224143. 15. Gao, W.; Wang, J., The Environmental Impact of Micro/Nanomachines: A Review. ACS Nano 2014, 8, 3170-3180. 16. Popovtzer, R.; Agrawal, A.; Kotov, N. A.; Popovtzer, A.; Balter, J.; Carey, T. E.; Kopelman, R., Targeted Gold Nanoparticles enable Molecular CT Imaging of Cancer. Nano letters 2008, 8, 4593-4596. 17. Gharagouzloo, C. A.; McMahon, P. N.; Sridhar, S., Quantitative contrast-enhanced MRI with superparamagnetic nanoparticles using ultrashort time-to-echo pulse sequences. Magnetic Resonance in Medicine 2015, 74, 431-441. 18. Bruchiel-Spanier, N.; Mandler, D., Nanoparticle-Imprinted Polymers: Shell-Selective Recognition of Au Nanoparticles by Imprinting Using the Langmuir–Blodgett Method. ChemElectroChem 2015, 2, 795-802. 19. Eiser, E., Dynamic Light Scattering. In Multi Length-Scale Characterisation, John Wiley & Sons, Ltd: 2014; pp 233-282. 20. Henriquez, R. R.; Ito, T.; Sun, L.; Crooks, R. M., The resurgence of Coulter counting for analyzing nanoscale objects. Analyst 2004, 129, 478-482. 21. Zhou, Y.-G.; Rees, N. V.; Compton, R. G., The Electrochemical Detection and Characterization of Silver Nanoparticles in Aqueous Solution. Angewandte Chemie 2011, 123, 4305-4307. 22. Tschulik, K.; Haddou, B.; Omanović, D.; Rees, N. V.; Compton, R. G., Coulometric sizing of nanoparticles: Cathodic and anodic impact experiments open two independent routes to electrochemical sizing of Fe3O4 nanoparticles. Nano Res. 2013, 6, 836-841. 23. Slocik, J. M.; Stone, M. O.; Naik, R. R., Synthesis of Gold Nanoparticles Using Multifunctional Peptides. Small 2005, 1, 1048-1052. 24. Kraus-Ophir, S.; Witt, J.; Wittstock, G.; Mandler, D., Nanoparticle-Imprinted Polymers for Size-Selective Recognition of Nanoparticles. Angewandte Chemie International Edition 2014, 53, 294-298. 25. Hitrik, M.; Pisman, Y.; Wittstock, G.; Mandler, D., Speciation of nanoscale objects by nanoparticle imprinted matrices. Nanoscale 2016, 8, 13934-13943. 26. Witt, J.; Mandler, D.; Wittstock, G., Nanoparticle-Imprinted Matrices as Sensing Layers for Size-Selective Recognition of Silver Nanoparticles. ChemElectroChem 2016, 3, 2116-2124. 27. Tokonami, S.; Shiigi, H.; Nagaoka, T., Review: Micro- and nanosized molecularly imprinted polymers for high-throughput analytical applications. Analytica Chimica Acta 2009, 641, 7-13. 28. Pichon, V.; Chapuis-Hugon, F., Role of molecularly imprinted polymers for selective determination of environmental pollutants—A review. Analytica Chimica Acta 2008, 622, 48-61. 29. Alexander, C.; Andersson, H. S.; Andersson, L. I.; Ansell, R. J.; Kirsch, N.; Nicholls, I. A.; O'Mahony, J.; Whitcombe, M. J., Molecular imprinting science and technology: a survey of the literature for the years up to and including 2003. Journal of Molecular Recognition 2006, 19, 106-180. 30. Datchary, W.; Mehner, A.; Zoch, H. W.; Lucca, D. A.; Klopfstein, M. J.; Ghisleni, R.; Grimme, D.; Brinksmeier, E., High Precision Diamond Machining of Hybrid Sol-Gel Coatings. Journal of Sol-Gel Science and Technology 2005, 35, 245-251.


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31. Vijayalakshmi, U.; Rajeswari, S., Synthesis and characterization of sol–gel derived glassceramic and its corrosion protection on 316L SS. Journal of Sol-Gel Science and Technology 2007, 43, 251-258. 32. Sugama, T., Cerium acetate-modified aminopropylsilane triol: A precursor of corrosionpreventing coating for aluminum-finned condensers. Journal of Coatings Technology and Research 2005, 2, 649-659. 33. Conde, A.; De Damborenea, J.; Durán, A.; Menning, M., Protective Properties of a SolGel Coating on Zinc Coated Steel. Journal of Sol-Gel Science and Technology 2006, 37, 79-85. 34. Sayilkan, H.; Şener, Ş.; Şener, E.; Sülü, M., The Sol-Gel Synthesis and Application of Some Anticorrosive Coating Materials. Materials Science 2003, 39, 733-739. 35. Parkhill, R. L.; Knobbe, E. T.; Donley, M. S., Application and evaluation of environmentally compliant spray-coated ormosil films as corrosion resistant treatments for aluminum 2024-T3. Progress in Organic Coatings 2001, 41, 261-265. 36. Sheffer, M.; Groysman, A.; Mandler, D., Electrodeposition of sol–gel films on Al for corrosion protection. Corrosion Science 2003, 45, 2893-2904. 37. Veeraraghavan, B.; Haran, B.; Slavkov, D.; Prabhu, S.; Popov, B.; Heimann, B., Development of a Novel Electrochemical Method to Deposit High Corrosion Resistant Silicate Layers on Metal Substrates. Electrochemical and Solid-State Letters 2003, 6, B4-B8. 38. Castro, Y.; Ferrari, B.; Moreno, R.; Durán, A., Silica Sol-Gel Coatings on Metals Produced by EPD. Journal of Sol-Gel Science and Technology 2003, 26, 735-739. 39. Shacham, R.; Avnir, D.; Mandler, D., Electrodeposition of Methylated Sol-Gel Films on Conducting Surfaces. Advanced Materials 1999, 11, 384-388. 40. Turkevich, J.; Stevenson, P. C.; Hillier, J., A study of the nucleation and growth processes in the synthesis of colloidal gold. Discussions of the Faraday Society 1951, 11, 55-75. 41. Bastús, N. G.; Comenge, J.; Puntes, V. c., Kinetically Controlled Seeded Growth Synthesis of Citrate-Stabilized Gold Nanoparticles of up to 200 nm: Size Focusing versus Ostwald Ripening. Langmuir 2011, 27, 11098-11105. 42. Shacham, R.; Mandler, D.; Avnir, D., Electrochemically Induced Sol–Gel Deposition of Zirconia Thin Films. Chemistry – A European Journal 2004, 10, 1936-1943.


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Figure Captions:

Figure 1: Schematics of the NAIM system: ITO substrate is treated with PDDA following by adsorption of AuNPs-cit. Then, the desired silane is electrodeposited on the electrode to form the NAIM. The AuNPs-cit are electrochemically removed by electrochemical oxidation to form the template, which is used to reuptake the AuNPs-cit

Figure 2: Cyclic voltammetry of 2 mM Fe(CN)63- in 0.1 M KCl for ITO electrodes modified by TEOS applying different depostion times: (A) with; (B) without preadsorbed AuNPs-cit.

Figure 3: EFTEM (energy filtered TEM) images of an Au-TEOS sample deposited for 120 s in (A) elastically scattered electrons only (zlf), (B) O K-edge 532 eV and (C) Si L-edge 99 eV signals. Notice that white means higher levels of the respective elements.

Figure 4: TEM image after electrodeposition of (A) 30 s, (B) 60 s and (C) 120 s of Au-TEOS samples.

Figure 5: CV of 0.5 mM ferrocenemethanol in 0.1 M KNO3 recorded with ITO electrodes electrochemically coated with TEOS/APTES for different deposition times. (A) with; (B) without preadsorbed AuNPs-cit.


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Figure 6: LSV of (A) Au-TEOS and of (B) Au-TEOS/APTES in 0.1 M KCl. The different curves represent different deposition times of the sol-gel matrix.

Figure 7: LSV of NAIMs that were formed by deposition of TEOS for: (A) 30; (B) 60 and (C) 120 s. The black curves show the oxidation of the originally imprinted AuNPs-cit; the red curves show the oxidation after reuptake of the nanoparticles for 1.5 h and the blue curves show the oxidation of non-imprinted matrices. The scan rate was 0.1 V s-1.

Figure 8: As Figure 7 but deposition of TEOS/APTES.

Figure 9: LSV of ITO electrochemically modified by TEOS (−1.0 V for 60 s) after the removal of the originally imprinted 8 nm AuNPs-cit, followed by immersing in either 8 or 30 nm AuNPscit.


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Figure 1

Figure 2


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Figure 3

Figure 4


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Figure 5

Figure 6

Figure 7


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Figure 8

Figure 9


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Table 1: The reuptake percent, which is the percent ratio between the charge of the reuptake oxidation wave and that of the original NAIMs of the TEOS matrix.

Deposition time [s]

Reuptake percentage [%]

30

45

60

25

120

10


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Electrochemically Deposited SolâGel Based Nanoparticle-Imprinted

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