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Core-Shell Nanoplasmonic Sensing for Characterization of Biocorona Formation and other Nanoparticle Surface Interactions Rickard Frost, Carl Wadell, Anders Hellman, Sverker Molander, Sofia Svedhem, Michael Persson, and Christoph Langhammer ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.6b00156 • Publication Date (Web): 16 May 2016 Downloaded from http://pubs.acs.org on May 17, 2016

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Core-Shell Nanoplasmonic Sensing for Characterization of Biocorona Formation and other Nanoparticle Surface Interactions

Rickard Frost1, 2, *, Carl Wadell2, Anders Hellman2, Sverker Molander1, Sofia Svedhem2, Michael Persson3, and Christoph Langhammer2, *

1

Department of Energy and Environment, Chalmers University of Technology,

SE-412 96 Göteborg Sweden 2

Department of Physics, Chalmers University of Technology, SE-412 96 Göteborg

Sweden 3

Akzo Nobel Pulp and Performance Chemicals, SE-445 80 Bohus, Sweden

*Corresponding Authors: Rickard Frost, e-mail: [email protected] Christoph Langhammer, e-mail: [email protected]

Abstract Surface properties of nanoparticles imposed by particle size, shape and surface chemistry are key features that largely determine their environmental fate and effects on biological systems. Consequently, development of analytical tools to characterize surface properties of nanomaterials and their relation to toxicological properties must occur in parallel with applications. As a

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contribution to this quest, we present a nanoplasmonic sensing strategy that enables systematic in situ characterization of molecule-nanoparticle interactions under well-controlled conditions, both in terms of nanoparticle size and surface chemistry, with particular focus on the importance of surface faceting in crystalline nanoparticles. We assess the performance of our sensing strategy by presenting two case studies: (i) Protein corona formation on facetted Au core SiO2 shell nanoparticles of different size, and thus different surface facet-to-edge ratios. Based on 2D and 3D models of the investigated structures, we find that for small particles the curved regions between adjacent facets dominate the response of the corona formation process, whereas the facets dominate the response in the large particle regime. (ii) In situ functionalization of Au core SiO2 shell nanoparticle surfaces, and analysis of the subsequent protein repellent behavior. Due to the versatility of the presented sensing strategy in studies of nanoparticle surface properties, including in situ surface modifications, and their interactions with (bio)molecules during corona formation, we foresee it to become a valuable tool in the areas of nanomedicine and nanotoxicology.

Keywords: nanoparticles, corona, indirect nanoplasmonic sensing, protein adsorption,

silanization,

facets,

core-shell


sensing

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The continuing expansion of nanotechnology in our society is associated with the production of an increasing number of engineered nanomaterials (ENMs), which hold great promise due to their unique physicochemical properties.1 As for chemicals in general, the increased production of ENMs leads to an increased exposure to both humans and to the environment. Therefore, it is imperative that this development is accompanied by appropriate risk assessments of ENMs.2,3 One key to safe development and use of ENMs is thorough characterization of their interactions with biological matter, e.g. proteins, membranes and cells.4-7 When introducing an ENM into a biological environment, e.g. the human body, its surface is inevitably coated with molecules, and a so-called corona is formed.8,9 The detailed properties of this corona are directly determined by specific ENM properties such as surface curvature, faceting/shape and surface chemistry. Subsequently, the corona largely determines the environmental fate, biological activity, and the toxicity of the ENM.10-12 Consequently, the development of analytical tools that address surface properties of ENMs and directly link them to the formation of a corona must keep pace, and occur in parallel with, the development and rapidly increasing production of ENMs.13,14 In particular, it is desirable to apply welldefined model systems for in situ surface analysis of the influence of particle size and shape on corona formation and of similar surface associated events (e.g., functionalization processes and their subsequent impact on the ENM properties). Today, a range of analytical techniques is applied to study the diverse interactions between proteins and nanoparticles.15 However, existing techniques are generally hampered by (i) the difficulty to address the dynamics


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of protein-nanoparticle interactions, e.g. to study the kinetics of corona formation and evolution, and (ii) by necessary sample purification steps prior to characterization, which might influence the formed corona and thus lead to erroneous data interpretation.16,17

In response to the above we present a sensing strategy based on plasmonic metal core – dielectric shell nanoparticles fabricated on a sensor “chip” surface, which enables in situ studies of the corona formation on dielectric ENMs in realtime (Figure 1). Using our methodology, it becomes possible to monitor both corona formation and evolution in biological environments in situ. Secondly, owing to the thin dielectric shell layer encapsulating the plasmonic core, the core-shell structure constitutes a mimic of dielectric ENMs. Compared to coreshell nanoparticles in suspension, which have previously been applied for sensing purposes18, our strategy however allows measurements under controlled flow conditions. Importantly, this means that nanoparticles on the chip can be modified in situ without the need of separate purification steps prior to characterization. This is of high importance since the purification process itself might influence the formed corona and thus information might be lost during sample preparation. Furthermore, the presented sensor chips are amendable towards high throughput analysis and lab-on-a-chip applications. We illustrate our plasmonic core-shell sensing strategy by presenting two different case studies: (i) Protein corona formation on SiO2-nanoparticle mimics of different size, for which we unearth the importance of different facet-to-edge ratios for different particle size. (ii) In situ characterization of SiO2-nanoparticle surface silanization and its influence on protein corona formation. We chose Au-


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silica core-shell structures because they are very relevant mimics of colloidal silica, which is industrially produced in large quantities and used in a wide range of industrial applications, e.g., in coatings and as polishing agent, as well as in papermaking.19

Figure 1. (A) Schematic representation of plasmonic core-shell nanoparticles on the sensor chip. The sphere-like facetted Au nanoparticles (size varied between different sensors) were fabricated by annealing low-aspect ratio truncated Au nanocones into facetted sphere-like particles (close to their equilibrium Wulff-shape20), and by subsequently coating them with a thin SiO2 layer. Each of the core-shell nanoparticles serves two functions. Firstly, the Au core constitutes a nanoplasmonic sensing element21 (optical nanotransducer) which allows the detection of surface functionalization and molecular adsorption (corona formation) events via tracking of spectral shifts of the plasmon resonance. Secondly, owing to the thin dielectric shell layer encapsulating the plasmonic core, the core-shell structure constitutes a mimic of dielectric ENMs in bulk solution. (B) Schematic representation of the experimental setup and plasmonic peak shift readout.

Experimental section Unless otherwise stated, all chemicals were obtained from commercial sources and used without further purification. Fused silica sensor substrates were obtained from Semiconducter Wafer Inc. (Hsinchu, Taiwan) and polystyrene


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colloids from Life Technologies Europe (Stockholm, Sweden). Deionized water was obtained from a Milli Q purification unit (Millipore, France) (resistivity 18.2 MΩ/cm). Phosphate buffered saline (PBS) was prepared from tablets (0.0015 M potassium dihydrogen phosphate, 0.0081 M disodium hydrogen phosphate, 0.0027 M potassium chloride and 0.137 M sodium chloride, pH 7.4) (Sigma Aldrich). Before use, all solutions were filtered and degassed. Bovine serum albumin

(BSA)

was

obtained

from

Sigma

Aldrich

and

gamma-

glycidoxypropyltriethoxysilane from AkzoNobel (Bohus, Sweden)

Nanofabrication and Nanoparticle Characterization The plasmonic sensor chips were fabricated by hole-mask colloidal lithography (HCL), described in detail elsewhere,22 followed by annealing (800°C, 24 h) and deposition of 10 nm SiO2 by plasma enhanced chemical vapor deposition (PECVD). The procedure is described in further detail in the Supporting Information (SI). The most important modification of the HCL process for the present work is that the evaporated Au-thickness was set to 80% of the mean diameter of the polystyrene colloids (dPS) used to fabricate the hole-mask. This with the purpose to have a low initial aspect ratio of the nanostructures to ease the structural rearrangement into the quasi-spherical facetted equilibrium shape during the subsequent annealing process. When imaged from the top (Figure 2AB), the characteristic amorphous (no long-range order) particle arrangement in the obtained array with average inter-particle distance on the order of three particle diameters is seen (coverage ≈ 10%). Moreover, the facetted particle surfaces after the annealing process become evident (Figure 2B).23 Figure 2C-H summarizes the structural rearrangement to the equilibrium shape of the


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differently sized Au-nanostructures. As can be seen from the tilted SEM images, the HCL-characteristic truncated cone-shaped Au structures formed during nanofabrication are transformed into more rounded, sphere-like but facetted structures, forming the cores of the targeted core-shell ENM mimics. In addition, since not easily observable using SEM, the height of the obtained nanostructures was measured by atomic force microscopy (AFM), as shown in Figure 2I-N. A detailed summary of the particle dimensions is presented in Table S1 in the SI. In the final step of the fabrication process a 10 nm thick layer of SiO2 was deposited by PECVD. The thickness of the shell layer (10 nm) was chosen such that plasmonic detection is efficiently possible at its outer surface via the enhanced local field.24


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Figure 2. Schematic depiction of the Au-nanostructures (A) before and (B) after annealing in air (800°C, 24 h) together with corresponding top view SEM images (x100 000). Note the hexagonal faceting present after annealing (scale bars: 200 nm). (C-E) SEM images (x250 000, tilt 75°) showing the three different sizes and the shapes of the Au-nanostructures (C-E) before and (F-H) after thermal annealing (scale bars: 200 nm). (I-K) AFM images of the nanostructures shown in F-H and (L-N) corresponding cross sections.

To further characterize the fabricated core-shell nanostructures with respect to shape and coverage of the SiO2-layer, TEM analysis was also performed. In Figure 3A-F top and side view TEM images of the core-shell structures are shown. It is evident that the particles are facetted and close to the equilibrium Wulff-shape20, however, influenced by the presence of a substrate (hence they


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are, for example, not symmetric). This implies that they exhibit both planar facets (predominantly (111) and (100)) and curved regions at edges and corners between adjacent facets.25,26 Furthermore, the TEM-images show a full coverage of the SiO2 layer, independent on the size of the Au cores. The targeted thickness of 10 nm is observed at the top surface of the Au core. In contrast, at the sides of the Au core the SiO2 shell is thinner due to shadowing effects and non-perfect step coverage of the PECVD deposition. Note also that the structures have been removed from the substrate and placed on a TEM grid to enable analysis. For this reason the bottom surface of the nanoparticles formerly in contact with the support is not covered by SiO2. The presence of facets is generic to all crystalline nanoparticles27 and thus a very important aspect in the quest of understanding ENM-interactions with biological systems. Nevertheless, with few exceptions28,29, this aspect is often overlooked in nanotoxicology studies. For example, nanoparticles are commonly approximated as spheres and the obtained data are interpreted in terms of global surface curvature. In view of the fact that crystalline nanoparticles are facetted, which means they exhibit flat and curved surfaces at length-scales comparable to individual biomolecules, it becomes clear that this is an oversimplification. Therefore, here, we analyze in detail the possible implications of this fact on the specific example of our facetted Au-SiO2 core-shell structures. The starting point of our discussion is that, when the size of a faceted nanoparticle changes, the ratio between planar facets and curved regions at edges and corners between adjacent facets will also change. However, since the angle between adjacent facets is constant if the overall shape is conserved (120° in our case, see Figure 3A-C) it is independent on nanoparticle size, as long as the


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overall (equilibrium) shape of the particles is the same. Thus, the curvature at edges and corners is basically constant (or at least very similar) for all facetted particle sizes. Thus we postulate that it is the ratio between flat facets and curved edges/corners that is the relevant parameter to consider for this class of nanoparticles and not the global curvature of an approximated sphere. To illustrate this we develop first a simple 2D model based on the 2D-projection of our core-shell structures, as obtained from the top view TEM-images shown in Figure 3A-C. We classify the circumference of our particles into two regions, i.e. flat and curved. The former corresponds to the facet and the latter to a crosssection through and edge between adjacent facets. The following statements about our case can be derived from this treatment: (i) Due to the hexagonal shape, the angles between adjacent facets are the same for all three particle sizes and thus also the curvature in this region. (ii) The curved regions are independent on particle size and span across ca. 25 nm, as detailed in Figure 3G. (iii) The length of the flat regions shrinks with particle size from ca. 44 nm to ca. 24 nm as also shown in Figure 3G. (iv) As a consequence of the above it becomes clear that it is the ratio between flat and curved regions is particle size dependent and not the curvature as such. Based on this analysis one can predict three different regimes for the interactions of facetted nanoparticles with biomolecules during corona formation. The first one, applicable for larger nanoparticles, will be predominantly controlled by the interactions with the flat facets. The second one, applicable for smaller nanoparticles, will be predominantly controlled by the interactions with the particle edges and corners, which feature different (but still size independent) curvature than the flat facets and may interact differently


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with biomolecules such as proteins.30 The third regime then corresponds to intermediate particle sizes, where it is the ratio between flat and curved regions that controls the particle properties. To further corroborate this general picture, it is useful to validate it also in 3D. For that purpose we rely on the fact that perfectly Wulff-shaped Au nanocrystals exhibit a combination of (100) and (111) facets in equilibrium.25,26 The facets of Wulff-shaped Au nanoparticles have been verified by many experimental observations using HRTEM.31,32 Since our particles are grown on a surface, meaning that they will not be perfectly symmetrically Wulff-shaped (Figure 3DE), we investigate the situation for two extreme cases to establish the possible upper and lower boundaries for the size-dependence of the ratio between flat and curved areas in 3D, and compare these to our simple 2D model based on the hexagonal cross section. Specifically we derive this ratio for cubic (only (100) facets) and octahedral (only (111) facets) nanocrystals, respectively, assuming the length of the curved region at an edge equal to 25 nm (as derived from TEM images) for both cases. The result of this analysis is presented in Figure 3H together with the data from the 2D-model. Clearly, the general trends are the same with the only difference being a slight shape-dependence of the critical size where the curved-to-flat-ratio is equal to one.


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Figure 3 (A-F) To-scale top and side view TEM images of the three sizes of core-shell nanoparticles used in our study. In the side view images the arrows point towards the top of the nanoparticles (i.e. away from the facet formerly in contact with the support). All scale bars equal 20 nm. (G) 2D model to derive the flat-to-curved-surface ratio based on the projected hexagonal shape of the core-shell plasmonic sensing nanoparticles derived from the TEM images. The numbers correspond to the experimentally determined (from TEM) lengths of the flat and curved regions in the projected hexagonal structure. (H) Calculated ratios between flat and curved areas for 3D cubic ((100) facets only) and octahedral ((111) facets only) Au-nanoparticles, which constitute the possible upper and lower boundaries since our structures exhibit both types of facets, compared to the value obtained for the hexagonal 2D model. Nicely, the 2D model derived from the experimentally determined hexagonal projection of the particles, falls in between the cubic and octahedral 3D case, which confirms its validity.


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Results and Discussion Case Study (i): Protein Adsorption to Facetted SiO2-Nanoparticle Mimics This case study represents an important scenario where ENMs acquire a protein corona when exposed to a biological environment, as biomolecules adhere to their surface. An often neglected parameter in such studies is the faceting of nanoparticles. However, the properties of the corona at the planar facets and the edges/corners may be different.28 To demonstrate our surface-associated nanoplasmonic sensing strategy in this context, as well as to test our hypothesis of the importance of the ratio between flat and curved regions at facetted nanoparticle surfaces in corona formation, we have investigated the adsorption of bovine serum albumin (BSA) to the three types of sensor surfaces described earlier. Albumin (66 kDa) is the most abundant protein in the circulation system and common in lung tissue. The structure of BSA has been shown to have a strong similarity to the structure of human serum albumin (HSA), with 75.6% homology in the amino acid sequence.33 Thus, the shape of both HSA and BSA may be approximated by a solid equilateral triangle with sides of 8.0 nm and a thickness of 3.0 nm.34 We explicitly mention these parameters here since they are very relevant when picturing the interaction of these molecules with our nanoparticles, specifically with respect to the relative dimensions of the molecule and the flat and curved regions (i.e. that they are on the same order of magnitude). Previously, BSA has been shown to have a curvature-dependent adsorption onto spherical (and thus not facetted) silica nanoparticles (diameters varied between 15 and 165 nm) where the native conformation is better preserved at high curvatures.30 This behavior has also been observed for other


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small globular proteins and may be explained by the decreasing surface interactions at high curvatures.35 At a flat surface a larger area is accessible to the protein enabling a stronger interaction and thereby a greater perturbation of the protein structure. However, also when adsorbed to silica nanoparticles with high curvature (diameter of 9 nm), BSA has been reported to undergo conformational changes.36

We now turn to our plasmonic sensing measurements, which were carried out in an Xnano System (Insplorion AB, Sweden). All experiments were carried out at room temperature. The PBS was degassed by sonication at low pressure before use to avoid formation of air bubbles in the system. BSA was dissolved in PBS (1 mg/mL) right before use. A flow rate of 100 μL/min was applied. Typical extinction spectra obtained for the three types of sensors used are shown in Figure 4A. As shown in Figure 4B, the peak shift during adsorption of BSA (1 mg/mL, 100 μl/min) is to the red and, on an absolute scale, larger for larger nanostructures due to the expected increase in sensitivity factor (see also Supporting Information). BSA interaction with silica particles has been extensively investigated and is known to adsorb even under non-favorable electrostatic conditions.37 Under the present conditions the protein adsorption is found to be mostly irreversible. As the next step of our analysis, in order to compare the formed protein coronas at the differently sized core-shell nanoparticles, we make use of our analysis of the sensor-specific parameters, m, the bulk sensitivity factor (i.e. peak shift (nm) per refractive index unit (RIU)) and ld, the decay length of the enhanced field. They were experimentally determined for all considered nanoparticle sizes, as


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detailed in the Supporting Information. Specifically, we use m, ld and the observed peak shift (Δλ) at t = 60 min, i.e. when the corona formation is welladvanced, to calculate its thickness (d) for a range of assumed refractive indices (nlayer) according to equation 1. Equation 1 was derived by solving equation S1 (Supporting Information) for the thickness of the protein layer.

(1)

This results in a plot of the type shown in Figure 4C, which displays all possible combinations of the corona thickness and refractive index that satisfy the experimentally determined condition of sensor peak shift for the three different particle sizes, according to equation 1. The obtained data can then be interpreted in the following two ways: 1. If we allow the assumption that the protein corona is homogenous across the surface of the nanoparticle (i.e. if the structural properties of the corona at planar facets and curved edges/corners are the same) the results indicate that the corona formed on the largest nanoparticles is thicker, or more optically dense, compared to the smaller nanoparticles. However, this assumption does most likely not hold since surface curvature is known to influence the structure of BSA upon adsorption to silica nanoparticles.30 Therefore, it is reasonable to expect that that the formed corona is different on flat facets compared to curved edges. 2. If we, on the other hand, assume that BSA forms coronas with different properties on the planar facets and the curved edge/corner regions, the results


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indicate that the response from the largest core-shell nanoparticles is dominated by BSA adsorption to flat regions. At these planar regions a larger proteinsurface interaction is possible, leading to a greater perturbation of the secondary structure of BSA. This generates a denser protein corona within the enhanced plasmonic field, compared to the curved regions. For this reason, the curve for the largest core-shell nanoparticles is shifted towards higher refractive indices in Figure 4C. On the contrary, the overlapping response of the smaller nanoparticles indicates that the curved regions dominate the response from BSA adsorption since they (i.e. their lateral extension and curvature) are very similar and independent on the size of the nanoparticles within the present size range, as discussed earlier. Moreover, this experimentally determined nanoparticle size at which this cross-over to the regime where curved regions dominate the corona formation process occurs, is in good agreement with our model, where it is expected to happen for particle sizes around 100 nm. To further corroborate this interpretation, we carry out one more analysis step. From the structure of BSA it is reasonable to believe that the protein adsorbs flat onto the surface of the nanoparticles for maximum surface interactions. Thus, in this configuration, this protein would generate a corona with a thickness of about 3 nm or less, depending on the degree of denaturation. The individual molecules would then extend around 8 nm laterally along the nanoparticle surface. Using these assumptions together with our 2D facet model then allows us to graphically test our hypothesis of a transition between two regimes of corona formation. In Figure 4D, we simply draw the number of BSA molecules that are expected to fit on the flat and curved areas, assuming slightly higher denaturation on the flat areas due to stronger surface interaction compared to


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the curved one.30 It indeed becomes clear that for the smallest particle size, a larger number of molecules is located on the curved areas, compared to the flat ones, corroborating our general concept and the specifics of our experimental findings in Figure 4C.

Figure 4. (A) Extinction spectra of the bare core-shell nanoparticles. (B) Real-time responses during the adsorption of BSA to the core-shell nanoparticles of different size and subsequent buffer rinse. The average response and standard deviation are indicated after 20, 40, 60, and 80 min. Each experiment was repeated three times. (C) The thickness of the adsorbed protein adlayers, at t = 60 min, calculated for a range of refractive indices. (D) Schematic of BSA adsorbed to flat and curved regions of the nanoparticle surfaces.

Case Study (ii): In Situ Functionalization of SiO2-Nanoparticles This second case study represents another important aspect of ENMs, namely chemical surface modification of nanoparticles after their synthesis, in order to achieve a specific function38,39, e.g. passivation of the surface through


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PEGylation40, or the attachment of targeting agents to nanomedicines41. Silica colloids are today used in many industrial applications (e.g. papermaking and surface polishing) and commercial products (e.g. coatings and adhesives) and thus technologically very relevant ENM model system for our purpose.19 The surface chemistry of silica nanoparticles can efficiently be tailored by silanization. Here we apply this strategy to demonstrate the in situ monitoring of a surface functionalization process, and the subsequent assessment of its effect on interactions with biomolecules during corona formation in one and the same experiment, i.e. without an ex situ purification process after surface functionalization, which is necessary in similar studies reported in the literature.42 Specifically, we silanized the core-shell sensor SiO2 surfaces with gamma-glycidoxypropyltriethoxysilane (Scheme S1, Supporting Information) with the purpose to suppress BSA adsorption. Prior to in situ silanization of the sensor surfaces, these were rehydroxylized to increase the number of silanol functional groups. This was performed by incubation for 90 min in an aqueous solution of 500 ppm silica sol (final pH 1011) at 50°C. The sensors were then rinsed with water and dried in a flow of N2. Gamma-glycidoxypropyltriethoxysilane was hydrolyzed by mixing 59 wt% silane with 41 wt% water (pH 2, adjusted using HCl) for 2 h. After hydrolyzation, the silane was used within one week. Just prior to use, the silane was diluted x100 in water and the pH was adjusted to 9.7 using NaOH.

The corresponding results are shown in Figure 5A and indicate that the obtained degree of surface silanization, whose kinetics are monitored in situ, basically completely prevents adsorption of BSA. A more direct comparison of the


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adsorption kinetics of BSA to the bare Au core - SiO2 shell surface and the silanized one is shown in Figure 5B and confirms the obtained protein-repellent properties after silanization.

It is now also interesting to briefly look at the obtained silanization kinetics in Figure 5A.

It is evident that there are two different regimes with large

differences in binding rate. In the first regime (I), a fast peak red-shift is observed, i.e. the reaction process occurs rapidly. In the second regime (II), the reaction proceeds more slowly. The observed data agree well with a previously suggested reaction model where first a single siloxane bond is formed between the silane molecule and the silica surface (primary reaction) that is followed by a slower condensation reaction between silanol groups at neighboring silane molecules (secondary reaction).43 To further analyze our data, we follow the same line as for case (i) and use the measured peak shifts after the first regime (I), and after the second regime (II) and rinse (=total response) as input to equation 1 to derive the adlayer-thickness vs. adlayer refractive index plot shown in Figure 5C. Here, the thickness of the formed silane adlayer is known to be equal to the length of the silane molecule. Thus, the adlayer thickness can easily be estimated to 1.23 nm from the lengths of the atomic bonds in the used silane molecule. With this number at hand, it is straightforward to derive the corresponding effective refractive indices of the silane adlayer in the fast (I - RII = 1.403) and slow (II - RIII = 1.490) silanization regimes from the plot in Figure 5C.


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Figure 5. (A) Nanoplasmonic sensing data showing the silanization process of 167 nm core-shell nanoparticles, and their subsequent exposure to BSA (1 mg/mL). After the surface functionalization the sensor was rinsed with water, followed by a medium exchange to PBS. The subsequent addition of BSA was performed in the same way as in case (i). (B) Comparison of observed responses of BSA adsorption to bare (black line) and silanized (grey line) core-shell nanoparticles. Note that the schematics are not drawn to scale. (C) Effective refractive indices of the silane adlayer determined after the fast (I) and total silanization process (II). Since no rinse was performed after the fast regime the observed peak shift at (I) was compensated for a change in bulk refractive index (-0.15 nm), measured for the change between water and silane solution.

These values can now be used to estimate the surface coverage of the bound silane in the different regimes according to the following procedure. To estimate


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the theoretical maximum coverage (θ) of a silane surface ligand on the SiO2 nanoparticle surface, the bulk number density of the molecule (N) is calculated according to equation 2 and multiplied with the thickness of the formed adlayer (d) according to equation 3. (2) (3) In equation 2, ρ is the density, M the molecular weight of the surface-bound molecule and NA is Avogadro´s number. A theoretical maximum coverage of 4.7 molecules/nm2 can be calculated from the density (1.35 g/cm3), refractive index (1.52) and length (1.23 nm) of the activated silane. The former two parameters were calculated using the software Chemsketch while the latter one was estimated from the lengths of the atomic bonds. It should be noted that the maximum coverage was calculated from the bulk number density and the length of the silane, not taking into account the silanol number (αOH) (i.e. the number of OH groups per unit surface area) at the surface of the SiO2 nanoparticle mimics. In the literature, it has been established that a fully hydroxylized SiO2 surface has a silanol number of 4.6 OH/nm2 (least squares method) and 4.9 OH/nm2 (arithmetical mean).44 Thus, the theoretical maximum surface coverage (4.7 molecules/nm2) is similar to αOH and was therefore considered valid. This number can now be compared with the one we can derive from our experiment by simply comparing the determined effective refractive indices after stage I and II, with the nominal refractive index of the surface ligand to obtain the respective coverage after stage (I) and (II) of 1.8 and 4.0 molecules/nm2, respectively. It should be noted that these calculations do not account for a change in refractive index due to possible crosslinking of adjacent silanes in the slow reaction regime.


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Such reactions would not alter the surface coverage of silane molecules. Considering the reaction kinetics of the silanization process in Figure 5A it is evident that the maximum response is not yet reached when interrupted. For this reason, an even higher coverage and/or degree of crosslinking may be achieved if a longer incubation time is allowed.

Future Prospects of the Nanoplasmonic Sensing Strategy At the general level, the presented case studies indicate that probing proteinnanoparticle interactions using our sensing strategy enables investigations of a potentially

large

variety

of

experimental

domains

relevant

to

the

nanotoxicological assessment of ENMs. Parameters that may be addressed include, e.g., the role of pH, protein concentration, and temperature on the corona formation. The exposure of specific epitopes after protein adsorption may also be investigated, in the same experiment, by subsequent addition of antibodies targeted towards these epitopes. Furthermore, the strategy enables studies of corona evolution, i.e. changes in corona properties upon changes in the surrounding biological environment in situ. Moreover, the strategy is generic in the sense that a wide range of materials such as oxides, nitrides, carbides, carbonaceous materials, as well as other metals can be grown as shells around a plasmonic core, facilitating the investigation of a very wide range of materials. Decreasing the thickness of the shell layer would increase the sensitivity of the sensors. For this reason, the thickness is an important parameter to consider for future optimization. However, when decreasing the thickness of the shell layer the risk for defects therein is increasing. For indirect nanoplamonic sensing a 10 nm shell layer, as applied in this study, is commonly used.45,46 Since the


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controlled

self-assembly

of

shape-selected

wet-chemically

synthesized

nanoparticles on surfaces also is rapidly developing, detailed investigations of the role of particle shape and faceting are also within reach.47 Based on these prospects, we foresee that nanoplasmonic sensing have the potential to contribute significantly to the continued development and risk/safety assessment of ENMs regarding environmental fate as well as in nanomedicine and nanotoxicology.

Conclusion We have developed a sensing strategy based on nanofabricated plasmonic metal core – dielectric shell nanoparticles, enabling in situ studies of nanoparticle surface functionalization and its impact on the corona formation. Such relationships are important in the context of safe design of engineered nanomaterials. Our strategy has the important advantage that with nanoparticles immobilized on the surface, no purification of the functionalized nanoparticles is needed prior to characterization since it can be carried out in situ. This is critical because the purification process itself might influence the formed corona and thus information might be lost during sample preparation prior to analysis. Here, we have illustrated our strategy using two different case studies: (i) Protein adsorption to nanofabricated facetted Au core - SiO2 shell nanoparticles with different size (different facet-to-edge ratio). (ii) In situ analysis of SiO2nanoparticle surface silanization and its influence on protein corona formation. For case (i), based on plasmonic sensing results and modeling of the nanoparticle size dependence of the flat surface facet to curved corner/edge ratio, we show that this ratio is the key parameter to consider, when correlating


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the specifics of corona formation with nanoparticle size. In case study (ii), by in situ

silanization

of

the

nanoparticle

surface

using

gamma-

glycidoxypropyltriethoxysilane, we show that BSA adsorption can be prohibited almost completely at a silane surface coverage of 4 molecules/nm2. At the general level, the presented case studies indicate that probing proteinnanoparticle interactions using our sensing strategy enables investigations of a potentially

large

variety

of

experimental

domains

relevant

to

the

nanotoxicological assessment of ENMs.

Acknowledgements This work was financially supported by the Chalmers Area of Advance in Nanoscience and Nanotechnology, the Swedish Research Council via project 2014-4956. Andreas Sundblom at AkzoNobel is gratefully acknowledged for fruitful discussions regarding silica chemistry. We acknowledge the Material Analysis Laboratory at Chalmers University of Technology for the use of their transmission electron microscope and for the support from the staff. Finally, we thank the Knut and Alice Wallenberg Foundation for their support of the infrastructure in the MC2 nanofabrication laboratory at Chalmers, and the Swedish Research Council for their support of the μ-fab cleanroom infrastructure in Sweden.

Supporting Information Available: The following files are available free of charge. R Frost et al – Revised Supporting Information Additional figures, including a schematic illustration of the nanofabrication procedure as well as data for determination of the sensitivity factors (m) and


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decay lengths (ld) of the sensors are presented. In addition, dimensions of the nanostructures determined by SEM and AFM as well as a reaction scheme of the SiO2 silanization process are included in the SI.

References (1) Roduner, E. Size matters: why nanomaterials are different. Chemical Society reviews 2006, 35, 583-592. (2) Nel, A.; Xia, T.; Madler, L.; Li, N. Toxic potential of materials at the nanolevel. Science 2006, 311, 622-627. (3) Elsaesser, A.; Howard, C. V. Toxicology of nanoparticles. Adv. Drug Del. Rev. 2012, 64, 129-137. (4) Nel, A. E.; Madler, L.; Velegol, D.; Xia, T.; Hoek, E. M. V.; Somasundaran, P.; Klaessig, F.; Castranova, V.; Thompson, M. Understanding biophysicochemical interactions at the nano-bio interface. Nat. Mater. 2009, 8, 543-557. (5) Rivera Gil, P.; Oberdorster, G.; Elder, A.; Puntes, V.; Parak, W. J. Correlating Physico-Chemical with Toxicological Properties of Nanoparticles: The Present and the Future. ACS Nano 2010, 4, 5527-5531. (6) Lynch, I.; Dawson, K. A. Protein-nanoparticle interactions. Nano Today 2008, 3, 40-47. (7) Wang, F. J.; Yu, L.; Monopoli, M. P.; Sandin, P.; Mahon, E.; Salvati, A.; Dawson, K. A. The biomolecular corona is retained during nanoparticle uptake and protects the cells from the damage induced by cationic nanoparticles until degraded in the lysosomes. Nanomedicine-Nanotechnology Biology and Medicine 2013, 9, 1159-1168. (8) Monopoli, M. P.; Aberg, C.; Salvati, A.; Dawson, K. A. Biomolecular coronas provide the biological identity of nanosized materials. Nat. Nanotechnol. 2012, 7, 779-786. (9) Walczyk, D.; Bombelli, F. B.; Monopoli, M. P.; Lynch, I.; Dawson, K. A. What the Cell "Sees" in Bionanoscience. J. Am. Chem. Soc. 2010, 132, 5761-5768. (10) Albanese, A.; Walkey, C. D.; Olsen, J. B.; Guo, H. B.; Emili, A.; Chan, W. C. W. Secreted Biomolecules Alter the Biological Identity and Cellular Interactions of Nanoparticles. ACS Nano 2014, 8, 5515-5526. (11) Schöttler, S.; Becker, G.; Winzen, S.; Steinbach, T.; Mohr, K.; Landfester, K.; Mailänder, V.; Wurm, F. R. Protein adsorption is required for stealth effect of poly(ethylene glycol)- and poly(phosphoester)-coated nanocarriers. Nat. Nanotechnol. 2016, 11, 372-377. (12) Landgraf, L.; Christner, C.; Storck, W.; Schick, I.; Krumbein, I.; Dahring, H.; Haedicke, K.; Heinz-Herrmann, K.; Teichgraber, U.; Reichenbach, J. R.; Tremel, W.; Tenzer, S.; Hilger, I. A plasma protein corona enhances the biocompatibility of Au@Fe3O4 Janus particles. Biomaterials 2015, 68, 77-88. (13) Nel, A. E.; Nasser, E.; Godwin, H.; Avery, D.; Bahadori, T.; Bergeson, L.; Beryt, E.; Bonner, J. C.; Boverhof, D.; Carter, J.; Castranova, V.; DeShazo, J. R.; Hussain, S. M.; Kane, A. B.; Klaessig, F.; Kuempel, E.; Lafranconi, M.; Landsiedel, R.; Malloy, T.; Miller, M. B.; Morris, J.; Moss, K.; Oberdorster, G.; Pinkerton, K.; Pleus, R. C.;


25

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 29

Shatkin, J. A.; Thomas, R.; Tolaymat, T.; Wang, A.; Wong, J. A Multi-Stakeholder Perspective on the Use of Alternative Test Strategies for Nanomaterial Safety Assessment. ACS Nano 2013, 7, 6422-6433. (14) Kelly, P. M.; Åberg, C.; Polo, E.; O'Connell, A.; Cookman, J.; Fallon, J.; KrpetićŽeljka; Dawson, K. A. Mapping protein binding sites on the biomolecular corona of nanoparticles. Nat. Nanotechnol. 2015, 10, 472-479. (15) Li, L. W.; Mu, Q. X.; Zhang, B.; Yan, B. Analytical strategies for detecting nanoparticle-protein interactions. Analyst 2010, 135, 1519-1530. (16) Sahneh, F. D.; Scoglio, C.; Riviere, J. Dynamics of Nanoparticle-Protein Corona Complex Formation: Analytical Results from Population Balance Equations. Plos One 2013, 8, e64690. (17) Khan, S.; Gupta, A.; Verma, N. C.; Nandi, C. K. Kinetics of protein adsorption on gold nanoparticle with variable protein structure and nanoparticle size. Journal of Chemical Physics 2015, 143, 164709. (18) Drescher, D.; Zeise, I.; Traub, H.; Guttmann, P.; Seifert, S.; Buechner, T.; Jakubowski, N.; Schneider, G.; Kneipp, J. In situ Characterization of SiO2 Nanoparticle Biointeractions Using BrightSilica. Advanced Functional Materials 2014, 24, 3765-3775. (19) Roberts, W. O. In Colloidal Silica Fundamentals and Applications, Bergna, H. E.; Roberts, W. O., Eds.; Taylor & Francis Group, 2006, pp 131-175. (20) Barmparis, G. D.; Lodziana, Z.; Lopez, N.; Remediakis, I. N. Nanoparticle shapes by using Wulff constructions and first-principles calculations. Beilstein Journal of Nanotechnology 2015, 6, 361-368. (21) Mayer, K. M.; Hafner, J. H. Localized Surface Plasmon Resonance Sensors. Chemical Reviews 2011, 111, 3828-3857. (22) Fredriksson, H.; Alaverdyan, Y.; Dmitriev, A.; Langhammer, C.; Sutherland, D. S.; Zaech, M.; Kasemo, B. Hole-mask colloidal lithography. Advanced Materials 2007, 19, 4297-4302. (23) Barnard, A. S.; Young, N. P.; Kirkland, A. I.; van Huis, M. A.; Xu, H. F. Nanogold: A Quantitative Phase Map. ACS Nano 2009, 3, 1431-1436. (24) Rodriguez-Fernandez, J.; Pastoriza-Santos, I.; Perez-Juste, J.; de Abajo, F. J. G.; Liz-Marzan, L. M. The effect of silica coating on the optical response of submicrometer gold spheres. Journal of Physical Chemistry C 2007, 111, 1336113366. (25) Barmparis, G. D.; Remediakis, I. N. Dependence on CO adsorption of the shapes of multifaceted gold nanoparticles: A density functional theory. Physical Review B 2012, 86, 085457. (26) Vitos, L.; Ruban, A. V.; Skriver, H. L.; Kollar, J. The surface energy of metals. Surface Science 1998, 411, 186-202. (27) Xia, Y. N.; Xiong, Y. J.; Lim, B.; Skrabalak, S. E. Shape-Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics? Angew. Chem. Int. Ed. 2009, 48, 60-103. (28) Miclaus, T.; Bochenkov, V. E.; Ogaki, R.; Howard, K. A.; Sutherland, D. S. Spatial Mapping and Quantification of Soft and Hard Protein Coronas at Silver Nanocubes. Nano Lett. 2014, 14, 2086-2093. (29) Gagner, J. E.; Qian, X.; Lopez, M. M.; Dordick, J. S.; Siegel, R. W. Effect of gold nanoparticle structure on the conformation and function of adsorbed proteins. Biomaterials 2012, 33, 8503-8516.


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Page 27 of 29

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(30) Roach, P.; Farrar, D.; Perry, C. C. Surface tailoring for controlled protein adsorption: Effect of topography at the nanometer scale and chemistry. J. Am. Chem. Soc. 2006, 128, 3939-3945. (31) Uchiyama, T.; Yoshida, H.; Kuwauchi, Y.; Ichikawa, S.; Shimada, S.; Haruta, M.; Takeda, S. Systematic Morphology Changes of Gold Nanoparticles Supported on CeO2 during CO Oxidation. Angew. Chem. Int. Ed. 2011, 50, 10157-10160. (32) Quintana, M.; Ke, X.; Van Tendeloo, G.; Meneghetti, M.; Bittencourt, C.; Prato, M. Light-Induced Selective Deposition of Au Nanoparticles on Single-Wall Carbon Nanotubes. Acs Nano 2010, 4, 6105-6113. (33) Majorek, K. A.; Porebski, P. J.; Dayal, A.; Zimmerman, M. D.; Jablonska, K.; Stewart, A. J.; Chruszcz, M.; Minor, W. Structural and immunologic characterization of bovine, horse, and rabbit serum albumins. Mol. Immunol. 2012, 52, 174-182. (34) He, X. M.; Carter, D. C. Atomic-structure and chemistry of human serumalbumin. Nature 1992, 358, 209-215. (35) Shemetov, A. A.; Nabiev, I.; Sukhanova, A. Molecular Interaction of Proteins and Peptides with Nanoparticles. ACS Nano 2012, 6, 4585-4602. (36) Clark, S. R.; Billsten, P.; Elwing, H. A fluorescence technique for investigating protein adsorption phenomena at a colloidal silica surface. Colloids Surf. B. Biointerfaces 1994, 2, 457-461. (37) Giacomelli, C. E.; Norde, W. The adsorption-desorption cycle. Reversibility of the BSA-silica system. J. Colloid Interface Sci. 2001, 233, 234-240. (38) Mout, R.; Moyano, D. F.; Rana, S.; Rotello, V. M. Surface functionalization of nanoparticles for nanomedicine. Chemical Society reviews 2012, 41, 2539-2544. (39) Sperling, R. A.; Parak, W. J. Surface modification, functionalization and bioconjugation of colloidal inorganic nanoparticles. Philosophical Transactions of the Royal Society A-Mathematical Physical and Engineering Sciences 2010, 368, 1333-1383. (40) He, Q. J.; Zhang, J. M.; Shi, J. L.; Zhu, Z. Y.; Zhang, L. X.; Bu, W. B.; Guo, L. M.; Chen, Y. The effect of PEGylation of mesoporous silica nanoparticles on nonspecific binding of serum proteins and cellular responses. Biomaterials 2010, 31, 1085-1092. (41) Gu, F. X.; Karnik, R.; Wang, A. Z.; Alexis, F.; Levy-Nissenbaum, E.; Hong, S.; Langer, R. S.; Farokhzad, O. C. Targeted nanoparticles for cancer therapy. Nano Today 2007, 2, 14-21. (42) Jana, N. R.; Earhart, C.; Ying, J. Y. Synthesis of Water-Soluble and Functionalized Nanoparticles by Silica Coating. Chemistry of Materials 2007, 19, 5074-5082. (43) Goerl, U.; Hunsche, A.; Mueller, A.; Koban, H. G. Investigations into the silica/silane reaction system. Rubber Chemistry and Technology 1997, 70, 608623. (44) Zhuravlev, L. T. The surface chemistry of amorphous silica. Zhuravlev model. Colloids and Surfaces A-Physicochemical and Engineering Aspects 2000, 173, 1-38. (45) Langhammer, C.; Larsson, E. M.; Kasemo, B.; Zoric, I. Indirect Nanoplasmonic Sensing: Ultrasensitive Experimental Platform for Nanomaterials Science and Optical Nanocalorimetry. Nano Lett. 2010, 10, 3529-3538. (46) Larsson, E. M.; Langhammer, C.; Zoric, I.; Kasemo, B. Nanoplasmonic Probes of Catalytic Reactions. Science 2009, 326, 1091-1094.


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(47) Diaz Fernandez, Y. A.; Gschneidtner, T. A.; Wadell, C.; Fornander, L. H.; Lara Avila, S.; Langhammer, C.; Westerlund, F.; Moth-Poulsen, K. The conquest of middle-earth: combining top-down and bottom-up nanofabrication for constructing nanoparticle based devices. Nanoscale 2014, 6, 14605-14616.


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For TOC only


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