Functional Group Coverage and Conversion Quantification in

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Functional Group Coverage and Conversion Quantification in Nanostructured Silica by 1H NMR Carina I. C. Crucho, Carlos Baleizaõ ,* and José Paulo S. Farinha* CQFM, Centro de Química-Física Molecular, and IN, Institute of Nanoscience and Nanotechnology, Instituto Superior Técnico, Universidade de Lisboa, Lisboa, Portugal

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S Supporting Information *

ABSTRACT: Silica nanostructured materials are important in many fields, including catalysis, imaging, and drug delivery, mainly due to the versatility of surface functionalization that can bestow a huge variety of chemical and physical properties. With most applications requiring precise control over this surface modification, characterization of surface composition and reactivity have become of extreme importance. We present a novel approach to track silica surface modification and quantify functional group coverage using only solution NMR. We test the method using different types of silica nanoparticles and surface modifications, to show that after dissolving the silica matrix, the 1H NMR spectra can be resolved for every single component of the mixture. By using an internal standard, we are able to quantify the density of ligands and follow their sequential modification. Our work presents a fast, accurate, and straightforward method for surface characterization of silica nanostructures, using widely available NMR spectroscopy and small amounts of sample.

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organic molecules on solid-phase samples.13 Infrared (IR) spectroscopy can be used to identify functional groups on the surface of SiNPs and to track multistep functionalization. However, it is hard to quantify the density of surface functional groups and the technique is not intrinsically surface-sensitive, sampling deep into the NP core. Thermogravimetric analysis (TGA) and elemental analysis (EA) provide quantitative information on silica functionalization, but cannot accurately identify and quantify the surface composition. In addition, in TGA the silica matrix decomposes from 300 to 700 °C (thermal decomposition of surface hydroxyl groups), overlapping the decomposition of organic functional groups, which complicates this analysis.14 Solid-state (SS) NMR is widely used to confirm the surface functionalization of SiNPs. In particular, using 29Si and 13C cross-polarization (CP) and direct polarization (DP) excitation methods with magic angle spinning (MAS) allows one to obtain qualitative and quantitative information on surface composition.15 Although the Qn and Tn resonances (from the condensation of TEOS and organically modified silanes, respectively) can be simultaneously detected, the signal intensities are not strictly proportional to the amount of surface bound ligands for the CP-MAS SS NMR.16 In addition, solid-state NMR methods usually result in broad peaks (due to restricted mobility of the species), require large amounts of sample, and have to be carried out on dried powders.

ilica nanostructure materials, and in special silica nanoparticles (SiNPs), combine a unique array of properties including large surface area, tunable porosity and surface chemistry, low toxicity, biocompatibility, and scalable synthetic availability.1 This versatility have attracted interest in many areas, including catalysis,2 drug delivery,3 imaging,4 and coating applications.5 However, bare SiNPs present applicability problems, such as limited colloidal stability in some solvents, possible low chemical stability in aqueous environments6 and hemolysis of red blood cells.7 Such issues are usually solved by surface functionalization of the SiNPs through the wellestablished siloxane chemistry.1,8 SiNPs are obtained by sol−gel chemistry and consist of an amorphous silica network that can be compact or highly porous. The so-called Stöber silica nanoparticles9 (SSNs) have a compact matrix and are usually obtained by the Stöber method10 or by reverse (water in oil) microemulsion.11 On the other hand, mesoporous silica nanoparticles (MSNs)1 are mostly an array of (either aligned or disordered) mesochannels, synthesized by a templated sol−gel approach.1 They feature a porous structure with large surface area (∼1000 m2 g−1) and two different accessible areas for independent chemical modification, the external particle surface and the interior pore wall surface. Moreover, silica (compact or porous) is also often used as a coating for other types of NPs resulting in hybrid nanomaterials.12 The increased complexity of hybrid silica nanostructures featuring multiple functionalities, goes hand in hand with the need for suitable and appropriate characterization methodologies. Qualitative and quantitative characterization of small molecules immobilized on the surface of NPs is a very challenging task, requiring the analysis of small amounts of © 2016 American Chemical Society

Received: August 10, 2016 Accepted: December 6, 2016 Published: December 6, 2016 681

DOI: 10.1021/acs.analchem.6b03117 Anal. Chem. 2017, 89, 681−687

Article

Analytical Chemistry

(TESPN), and trimethoxy(7-octen-1-yl)silane (TMOenS), to yield amine, thiol, carboxylic acid (upon hydrolysis of the nitrile group) and alkene functionalized materials. These versatile functional groups allow several conjugation possibilities through linker chemistry.1 Before measuring the 1H NMR spectra, SiNPs in deuterated water are treated with NaOH to dissolve the silica network and release the surface-bound groups into solution. The spectra obtained for treated SSNs are presented in Figure 2. All the

In contrast, solution NMR methods provide excellent spectral resolution, but surprisingly, they have been scarcely used in the characterization of functionalized colloids. This is probably because species anchored to nanoparticles have reduced mobility, and this severely restricts their detection.17 Here we report the application of solution proton NMR to the characterization of SiNPs functionalized with several small organic groups. Specifically, we are able to identify and quantify the covalently bound ligands by a simple strategy that combines dissolution of the SiNPs and standard NMR experiments. We also expanded the applicability of this methodology to monitor functional group conversion at the surface of SiNPs and compare the results of our method with those of TGA and EA. Overall, our novel method provides quantitative information on the surface modification of silica nanomaterials using the widely available fast and simple H1−NMR technique.

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RESULTS AND DISCUSSION Synthesis and Characterization of Surface-Functionalized SiNPs. Silanol groups can be easily modified to tune the functionality of the SiNPs surface. However, methodologies for a fast, precise and quantitative determination of surface coverage are still limited. In order to check our novel solution-NMR based technique, we prepared two types of amorphous SiNPs: mesoporous silica nanoparticles (MSNs) and nonporous (Stöber) silica nanoparticles (SSNs). Both were surface-modified with four simple silane derivatives to cover a diversity of chemical properties (Figure 1). A typical functionalization reaction is schematically shown in Supporting Information (Figure S1).

Figure 1. Schematic representation of the functionalized SiNPs used in this work. All the organosilane molecules were grafted onto the surface.

The monodisperse nonporous SSNs were prepared using a modified ammonia-catalyzed Stöber method, with an average diameter of (20 ± 3) nm obtained by TEM (Figure S2, Supporting Information). The MSNs were synthesized by the liquid-crystal templating approach, using CTAB as structure-directing agent and NaOH as catalyst, under dilute aqueous conditions,18 with an average diameter of (134 ± 20) nm obtained by TEM (Figure S3, Supporting Information). The presence of the mesostructure was confirmed by powder X-ray diffraction, which yielded the typical pattern of mesoporous silica with hexagonal arrangement (Figure S4, Supporting Information). The MSNs have a BET surface area of 1080 m2 g−1, and BJH pore size of 2.8 nm (obtained from nitrogen sorption isotherms, Figure S5, Supporting Information). The surface of SiNPs was covalently modified with 3aminopropyltriethoxysilane (APTES), 3-mercaptopropyl-trimethoxysilane (MPTMS), 3-(triethoxysilyl)propionitrile

Figure 2. Solution 1H NMR (at pH = 13) with peaks assigned for the inset molecules. SSNs were functionalized with (A) APTES, (B) MPTMS, and (C) TESPN (hydrolyzed to carboxylic acid). Residual ethanol protons are denoted by (*).

peaks have been assigned and associated with specific atom positions in the structure (Supporting Information). The absence of possible silica fragments that could affect the measurements was verified by dynamic light scattering, with no particles being observed. Starting with hydrophilic groups, for the APTES-functionalized sample (Figure 2A, peaks of the protons of the functional group identified on the inset structure), we observe that the ethoxy group yields distinct resonances at 1.2 and 3.6 ppm. These peaks correspond to 682


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Figure 3. Solution 1H NMR (at pH = 13) with peaks assigned for the inset molecules. MSNs were functionalized with (A) APTES, (B) MPTMS, and (C) TMOenS. Residual ethanol protons are denoted by (*).

end, we grafted TMOenS on the surface of MSNs, obtaining an alkene functionality that is poorly soluble in aqueous basic media. In this case, after dissolving the silica network with NaOH only the solvent peaks were observed in the NMR spectrum. However, after we added a small amount of DMSOd6 to the NMR tube to solubilize the hydrophobic component, the 1H NMR spectrum show the typical peaks for the hydrocarbon chain, and also the vinylic protons at their characteristic chemical shift (Figure 3C). The methoxy 1H NMR peak is missing from the spectra of MPTMS and TMOenS-functionalized MSNs (Figure 3B,C) due to hydrolysis during the template removal (a step inexistent in the case of SSN). The peaks of the protons from the alpha-carbon to the silicon atom (less affected by possible interactions/transformations of the functional group) were used to estimate the amount of organic molecules grafted onto the surface of the SiNPs, by comparing their integrated intensities with that of 1,3,5-trioxane, used as internal standard (Table 1). To validate our method, the amount of organic material incorporated in the SiNPs was also determined by TGA. Figure 4 presents the TGA results and corresponding derivative plots (DTG) for several functionalized SSNs and MSNs. The initial weight loss at temperatures lower than 150 °C is attributed to the loss of residual ethanol and water (Figure 4). This initial weight loss is followed by an abrupt drop in weight due to the decomposition of organic groups, with the shape of the curve derivatives depending on the grafted organic functional group and on the SiNP type. MSN@NH2, SSN@NH2 and SSN@ COOH show a gradual weight loss curve at temperatures > 150

resonances of residual ethanol from washing the SSNs after synthesis (solvent remaining entrapped on the silica matrix after drying), and indicate possible incomplete condensation of the alkoxysilane groups. Similar resonance peaks were found in the NMR spectrum of SSN@SH (Figure 2B), corresponding to residual washing ethanol and methoxy groups resulting from incomplete alkoxysilane condensation (3.3 ppm). In the NMR spectrum of SSN@COOH (Figure 2C), the methylene protons next to the carboxylic group have a complex pattern that arises not only from the incomplete hydrolysis reaction of the nitrile group, but also from two different environments due to the protonation states of the carboxylic group. Again, residual washing ethanol and incomplete alkoxysilane condensation are detected. In the case of MSNs, we functionalized the external particle surface before template removal. The solution NMR spectra of the functionalized MSNs were measured after removing the CTAB template with an acidified ethanolic solution and dissolving the silica network with base (Figure 3). The NMR spectra of APTES-functionalized MSNs is similar to the one observed for SSN@NH2 (Figure 3 A). On the other hand, in the NMR spectrum of MSN@SH (Figure 3B), the methylene protons adjacent to the thiol group have a complex pattern that did not appear in the spectrum of SSN@SH. Since the functionalization steps for MSNs and SSNs diverge only in the surfactant removal step in the case of MSN@SH, we believe this step is promoting side reactions such as oxidation of thiol. Following the successful characterization of hydrophilic functional groups in SiNPs, we attempted the characterization of a hydrophobic surface modification by our method. To this 683


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Supporting Information). Once more, solution NMR is in agreement with EA results. We also attempted the characterization of SiNPs functionalized with two different groups. To this end, we grafted APTES and MPTMS onto the surface of SSNs. The NMR spectra of SSN@NH2@SH is similar to the one observed for SSN@NH2 and SSN@SH (Figure S7, Supporting Information). Due to the different chemical shift of the protons of the alpha-carbon to the silicon atom, the NMR peaks were sufficiently resolved for quantification by proton NMR (Table S1, Supporting Information). The results were found to be in good agreement with EA (Table S1, Supporting Information). Our novel solution NMR procedure for the precise quantification of functional groups on SiNPs have in fact several advantages over TGA and other techniques. Not only are solution phase NMR spectrometers widely available, but also the proposed method requires shorter experiment times and smaller amounts of material than currently used procedures. Surface functionalization with organosilane molecules occurs by reaction with the silica surface hydroxyl groups, being inherently limited by hydroxyl content which usually varies in the range of 3−7 SiOH/nm2.19 If we assume that an organosilane molecule with three alkoxide groups reacts with up to three Si−OH surface sites, the maximum number of functional groups would be in the range of 1−3 molecules/nm2. The values obtained by solution 1H NMR (Table 1) are in good agreement with this upper limit, showing good control of the surface modification procedure. Finally, to show the need to dissolve the SiNPs before NMR measurements, we also measured the NMR spectra of intact SiNPs dispersed in D2O (without dissolving the silica network), but in this case, we were only able to detect the APTES surface

Table 1. Functional Group Quantification for SiNPs by NMR and TGA sample

NMR (mmol/g)

TGAa (mmol/g)

surface coverage (molecules/nm2)

MSN@NH2 MSN@SH MSN@vinyl SSN@NH2 SSN@SH SSN@COOH

0.38 0.41 0.45 0.81 0.14 0.23

0.36 0.43 0.44 0.78 0.17 0.21

2 2 2 3 0.5 1

a

Corrected for the weight loss due to silica network condensation at high temperatures.

°C, while SiNPs functionalized with thiol groups (MSN@SH and SSN@SH), exhibited an abrupt weight loss at about 300 °C, with the same behavior being observed for MSN@vinyl. At higher temperatures the silica network can start to condensate (with water release), with this weight loss overlapping the organic group decomposition, thus impairing organic content quantification by TGA. To address this point, a control TGA experiment was performed with the bare particles (SSN and MSNs, Figure S6, Supporting Information), and the weight loss for temperatures above 150 °C were used to correct the curves of the functionalized SiNPs. This feature of TGA curves obtained for functionalized SiNPs is usually not addressed in the literature, which means that reported TGA functional group quantification in surface-modified SiNPs has been commonly overestimated (Table S1, Supporting Information). Following this procedure, the quantitative analysis of functionalized SiNPs by solution NMR is in excellent agreement with TGA results (Table 1). In addition, the results were further confirmed by elemental analysis (EA) and quantitatively estimated (Table S1,

Figure 4. TGA (red) and DTG (blue) of the thermal decomposition of SiNPs functionalized with APTES, MPTMS, TESPN, and TMOenS. The carboxylic acid was obtained upon hydrolysis of the nitrile group. 684


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Analytical Chemistry modification, and even in this case, recovering coverage values which are lower than those obtained for the dissolved SiNPs, due to the much lower mobility of the organic groups when covalently attached to the particles (Table S1, Supporting Information). Functional Group Conversion on Surface-Modified SiNPs. Surface modification of SiNPs often encompasses multiple steps, involving the conversion of the groups initially anchored to the silica surface into different functionalities. To extend the scope of the successful identification and quantification of covalently bound ligands on the surface of SiNPs, we decided to use our solution 1H NMR methodology to track the conversion of functional groups on the surface of SiNPs. To this end we prepared carboxylic acid modified particles, SSN@COOH and MSN@@COOH, by hydrolysis of SSN@CN and MSN@@CN particles using HCl (MSN@@ indicating modification of both internal and external surfaces), and sulfonic acid modified particles, MSN@@SO3H, by oxidation of MSN@@SH with hydrogen peroxide (Figure S8, Supporting Information). To obtain MSNs modified in both the external surface and the internal (pore) surface, we performed the functionalization after template removal (Figures S9 and S11, Supporting Information), with the mesostructure of the MSNs being maintained after the transformations (Figures S10 and S12, Supporting Information). The reaction conversions were followed by solution 1H NMR, and the spectra were collected until no further conversion was observed. Figure 5 shows the spectra of the different particles before the reaction started and at the end of the reaction. For the hydrolysis of SSN@CN (Figure 5A), the chemical shifts of the methylene protons adjacent to the silicon atom move to lower chemical shift, as the nitrile is converted to the carboxylic acid group. As before, the methylene protons next to the carboxylic group are split into two multiplets due to the protonation states of the carboxylic group. NMR integration shows that the conversion is almost 90%. For MSNs with inner and outer surface functionalization, the 1H NMR spectra (Figure 5B) is similar to that of SSN@CN (Figure 5A), however, the conversion to carboxylic acid is only 60% on the MSNs, due to the lower accessibility of the inner pore surface. Likewise, the same behavior was found on the oxidation reaction of thiol groups grafted on MSNs, which showed only a 53% conversion (Figure 5C). In this case, the methylene protons adjacent to the silicon atom do not shift significantly from MSN@@SH to MSN@@SO3H, due to larger distance of the functional group reducing its effect on the chemical shift.

Figure 5. Comparison of the 1H NMR (at pH = 13) spectra for functional group conversion into carboxylic groups (by hydrolysis of nitrile) on SSNs (A) and MSNs (B) and into sulfonic groups (by oxidation of thiol) on MSNs (C).

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EA and TGA (after correction for silica condensation at high temperature). The same methodology was used for the characterization of surface group conversion, having shown good results in tracking the hydrolysis of nitrile to carboxylic and the oxidation of thiol to sulfonic (in this case, even in the internal pore surface of MSNs). Our method presents several advantages over the methods conventionally used to characterize the surface modification of nanostructured silica materials (TGA, solid state NMR, elemental analysis): it requires very small amounts of sample; the measurements are simple and fast; and it uses widely available solution NMR instrumentation. This easy, fast, and accurate method can be easily applied to any other sol−gel silica structure with different organic (internal/external) surface

CONCLUSIONS We report a new method, based on solution 1H NMR, to quantify the functionalization of SiNPs and subsequent chemical transformations of these functional groups. The external surface of both compact and mesostructured SiNPs were modified with organic alkoxysilanes containing functional groups with different polarity. Using solution 1H NMR we were able to quantify the amount (or coverage) of the organic silane immobilized on the particles. The in situ destruction of the silica network at high pH allow the organic molecules to acquire the necessary mobility for NMR detection, with hydrophobic groups requiring only the addition of a suitable cosolvent to the aqueous media. The results of the proposed methodology are in excellent agreement with those obtained by 685


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bath at 80 °C. TEOS (2.5 mL) was added dropwise to the surfactant solution. The reaction was allowed to proceed for 2 h. Then, the heating was stopped and the suspension was cooled to room temperature. This solid crude product was collected by centrifugation (30000 G, 15 min) and washed first with a mixture of ethanol/water (30000G, 15 min) and twice with ethanol (30000 G, 15 min). The particles were kept in an oven for 24 h at 50 °C and dried in vacuum for 24 h. Surface Functionalization of SiNPs. The organo alkoxysilane functionalization was performed by postsynthesis grafting method. In a typical functionalization procedure, into a 25 mL round-bottom flask, 250 mg of SiNPs and 9 mL of anhydrous toluene were added under an inert atmosphere. This flask was sonicated for 15 min. Then, the desired organosilane was added (2 or 4 molecules/nm2, whereas 40 reactant molecules/nm2 was used for the functionalization of the interior and outer surfaces (MSN@@CN and MSN@@SH)), and the solution was brought to reflux for 24 h. For the SSN@ NH2@SH we added in one pot 2 molecules/nm2 for APTES and/or 4 molecules/nm2 for MPTMS. After cooling to room temperature, the functionalized particles were separated from solution via centrifugation (15000 G, 15 min), washed three times with ethanol (15000 G, 10 min), and dried. The carboxylic acid group was obtained by nitrile hydrolysis (MSN@@CN or SSN@CN). A total of 300 mg of SiNPs was suspended in 15 mL of HCl 9 M. After sonication (15 min), the reaction was stirred 24 h at 30 °C. The particles were recovered via centrifugation and washed with water until the pH of the washings was neutral, then twice with ethanol, and dried. The sulfonic group was obtained by the oxidation of thiol-functionalized MSNs. Typically, 1 g of MSN@@SH was suspended in 7.5 mL of MeOH and sonicated for 3 min. Then, H2O2 30% (2.4 mL) was added and the suspension was stirred at room temperature for 24 h. The particles were separated by centrifugation (27000 G, 15 min) and washed once with EtOH/H2O (1:1). The oxidized material was resuspended in 80 mL of H2SO4 0.1 M for 4 h. Finally, the MSN@@SO3H particles were washed four times with water (20000 G, 15 min) and dried. Following MSNs surface functionalization, residual CTAB was removed by acid ethanol extraction. MSNs (0.150 g) were dispersed 15 min in an ultrasonicator bath in 6 mL of a 0.5 M HCl ethanolic solution. The resulting suspension was stirred overnight at 50 °C, followed by centrifuging at 15000 G for 15 min to separate the particles from the supernatant, and washing three times with ethanol (15000 G, 10 min). When functionalization of the interior and outer surfaces was desired (MSN@@CN and MSN@@SH), the surfactant was extracted prior to any functionalization. All the particles were dried first at 50 °C for 24 h and latter in vacuum for 24 h. NMR Spectroscopy. Solution proton NMR data were collected on a Bruker Avance III 400 spectrometer (Bruker BioSpin GmbH, Rheinstetten, Germany) operating at 400 MHz. Samples were prepared by dispersing 5 mg of SiNPs in 500 μL of D2O/NaOH (pH 13) for 10 or 5 min for thiol functionalized SiNPs. For the SSN@vinyl, an extra 100 μL of DMSO-d6 were added to the NMR tube in order to solubilize the hydrophobic component. The samples with the SiNPs dispersed in the 500 μL of D2O were sonicated for 1 h, and then the NMR experiment was performed immediately. The dissolution of the silica matrix was checked by DLS. A known amount of 1,3,5-trioxane (ca. 0.5 mg) was added as internal

modifications. We believe it can become a standard in the characterization of silica surface modification.

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EXPERIMENTAL SECTION Chemicals. N-Cetyltrimethylammonium bromide BioXtra 99% (CTAB), tetraethylorthosilicate 98% (TEOS), 3-aminopropyltriethoxysilane 98% (APTES), 3-(triethoxysilyl)propionitrile 97% (TESPN), 3-mercaptopropyltrimethoxysilane 98% (MPTMS), and trimethoxy(7-octen-1-yl)silane 80% (TMOenS) were purchased from Sigma-Aldrich. 1,3,5-Trioxane was purchased from Fluka. Absolute ethanol (EtOH) was purchased from Fisher Chemical. Toluene, aqueous ammonium hydroxide (25 wt %; NH4OH), and concentrated hydrochloric acid (HCl, 37%) were purchased from Sigma-Aldrich. Toluene was refluxed over calcium hydride for 24 h and then distilled prior to use. Water was purified using a Millipore Milli-Q system to a resistivity of 18.2 MΩ. Unless otherwise specified, all chemicals were used as received without further purification. Material Characterization. Particle morphology was characterized with a Hitachi 8100 transmission electron microscope (TEM) operating at 200 kV and a current of 20 μA. The SiNPs samples were suspended in EtOH with an ultrasonicator. Subsequently, samples were prepared by placing one drop of the SiNPs solution onto Formvar-coated copper grid. Excess solution was drained, and the sample was allowed to air-dry. The dry nanoparticle size was estimated by measuring the average diameter of 100 nanoparticles by using ImageJ software. The thermogravimetric analysis was performed in a Setaram TGA instrument with a heating rate of 5 °C/min. Samples were heated from room temperature to ∼800 °C under a flow of nitrogen. The samples weights were in the range 13−20 mg. Mass loss during the run was used to approximate the loading of the organic functional group. For the calculations it was assumed that all alkoxysilane derivatives were grafted on the SiNPs surface through three siloxane bonds, that is, losing three ethoxy/methoxy groups, except the thiol-functionalized SiNPs, for which two methoxy groups were subtracted in the concentration calculation. Pore structure/ordering information was obtained by smallangle X-ray scattering analysis (Rigaku, MiniFlex II) of the dry MSNs powder. The CuKα line (1.54 Å) was used as radiation source. The nitrogen adsorption desorption isotherm measurements were performed on a Gemini VI (Micromeritics Corp. Atlanta, GA) surface analyzer. The surface areas were calculated with Brunauer−Emmett−Teller (BET) theory. Synthesis of Stö ber Silica Nanoparticles. Uniform nonporous silica nanospheres were synthesized using a modified Stöber method. Briefly, in a 250 mL polypropylene bottle, ethanol (86 mL), Milli-Q water (9 mL), and NH4OH solution (25%, 1.5 mL) were mixed. The flask was put into an oil bath and heated to 50 °C. Then TEOS (4.5 mL) was added to the above solution, and the reaction proceeded at 50 °C for 48 h. Thereafter, the particles were collected by high-speed centrifuge (70000 G, 20 min) and washed with ethanol three times. The solid was kept in an oven for 24 h at 50 °C and dried in vacuum for 24 h to remove any residual solvents. Synthesis of Mesoporous Silica Nanoparticles. The porous silica nanoparticles were synthesized based on a previously described procedure.18 Typical, in a 500 mL polypropylene bottle, 0.5 g of CTAB was mixed in 240 mL of Milli-Q water. After a clear solution was obtained, NaOH aqueous solution (2.0 M, 1.75 mL) was introduced to the CTAB solution and the reaction flask was introduced in an oil 686


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Analytical Chemistry standard. All 1H chemical shifts were referenced to the residual solvent proton signal (a broad singlet centered at 4.79 ppm). The surface coverage (molecules per nm2) was calculated from the surface area and volume of a particle and using a density of 0.34 g/mL for MSN and 1.6 g/mL for SSNs. First, from the mean particle size, we calculated the area (A = 4πr2) and volume (V = 4/3πr3) of the particles. The mass of a particle was calculated from the density and the calculated volume, allowing the quantification of the number of particles per gram of material. With the number of particles per gram, and the area of an individual particle, we determined the total area (m2) per gram. The concentration of the functionalized compound (mol/m2) can then be obtained from the total area and the value obtained by TGA or NMR (mol/g). From the product between the total area (m2) and the concentration of the functionalized compound (mol/m2), we obtained the total number of moles of the functionalized compound and, from this, the corresponding number of molecules. By dividing the number of molecules by the total area, we obtain the number of molecules/nm2.

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(5) Ribeiro, T.; Baleizão, C.; Farinha, J. P. S Materials 2014, 7, 3881. (6) (a) He, Q.; Shi, J.; Zhu, M.; Chen, F. Microporous Mesoporous Mater. 2010, 131, 314. (b) Hoshikawa, Y.; Yabe, H.; Nomura, A.; Yamaki, T.; Shimojima, A.; Okubo, T. Chem. Mater. 2010, 22, 12. (7) (a) Slowing, I. I.; Wu, C.-W.; Vivero-Escoto, J. L.; Lin, V. S.-Y. Small 2009, 5, 57. (b) Lin, Y.-S.; Haynes, C. L. J. Am. Chem. Soc. 2010, 132, 4834. (8) (a) Graf, C.; Gao, Q.; Schütz, I.; Noufele, C. N.; Ruan, W.; Posselt, U.; Korotianskiy, E.; Nordmeyer, D.; Rancan, F.; Hadam, S.; Vogt, A.; Lademann, J.; Haucke, V.; Rühl, E. Langmuir 2012, 28, 7598. (b) Natarajan, S. K.; Selvaraj, S. RSC Adv. 2014, 4, 14328. (9) Tang, Li; Cheng, J. Nano Today 2013, 8, 290. (10) Stöber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62. (11) Bagwe, R. P.; Yang, C.; Hilliard, L. R.; Tan, W. Langmuir 2004, 20, 8336. (12) Piao, Y.; Burns, A.; Kim, J.; Wiesner, U.; Hyeon, T. Adv. Funct. Mater. 2008, 18, 3745. (13) Zhang, B.; Yan, B. Anal. Bioanal. Chem. 2010, 396, 973. (14) (a) Burleigh, M. C.; Markowitz, M. A.; Spector, M. S.; Gaber, B. P. Chem. Mater. 2001, 13, 4760. (b) Burleigh, M. C.; Markowitz, M. A.; Spector, M. S.; Gaber, B. P. J. Phys. Chem. B 2001, 105, 9935. (15) (a) Trebosc, J.; Wiench, J. W.; Huh, S.; Lin, V. S.-Y.; Pruski, M. J. Am. Chem. Soc. 2005, 127, 3057. (b) Davidowski, S. K.; Holland, G. P. Langmuir 2016, 32, 3253. (c) Rapp, J. L.; Huang, Y.; Natella, M.; Cai, Y.; Lin, V. S.-Y.; Pruski, M. Solid State Nucl. Magn. Reson. 2009, 35, 82. (16) (a) Suteewong, T.; Ma, K.; Drews, J. E.; Werner-Zwanziger, U.; Zwanziger, J.; Wiesner, U.; Bradbury, M. S. J. Sol-Gel Sci. Technol. 2015, 74, 32. (b) Grünberg, A.; Yeping, H.; Breitzke, G.; Buntkowsky, Z. Chem. - Eur. J. 2010, 16, 6993. (17) (a) Moreels, I.; Martins, J. C.; Hens, Z. Sens. Actuators, B 2007, 126, 283. (b) Lehman, S. E.; Tataurova, Y.; Mueller, P. S.; Mariappan, S. V. S.; Larsen, S. C. J. Phys. Chem. C 2014, 118, 29943. (c) Hristov, D. R.; Rocks, L.; Kelly, P. M.; Thomas, S. S.; Pitek, A. S.; Verderio, P.; Mahon, E.; Dawson, K. A. Sci. Rep. 2015, 5, 17040. (18) Lin, Y.-S.; Tsai, C.-P.; Huang, H.-Y.; KuHuo, C.-T.; Hung, Y.; Huang, D.-M.; Chen, Y.-C.; Mou, C. Y. Chem. Mater. 2005, 17, 4570. (19) Jung, H.-S.; Moon, D.-S.; Lee, J.-K. J. Nanomater. 2012, 2012, 593471.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.6b03117. SiNPs functionalization schemes; Nitrogen adsorption/ desorption isotherms and pore volume; TEM images of SiNPs and respective size histograms distribution; Powder X-ray diffraction patterns of MSNs; 1H NMR peaks assignments; Functional group quantification by aqueous NMR on the intact SiNPs and TGA values without correction (PDF).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

José Paulo S. Farinha: 0000-0002-6394-5031 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by Fundaçaõ para a Ciência e a Tecnologia (FCT-Portugal) and COMPETE (FEDER), Projects RECI/CTM-POL/0342/2012, RECI/QEQ-QIN/ 0189/2012, PTDC/CTM-POL/3698/2014, and UID/NAN/ 50024/2013. The authors thank Dr. Auguste Fernandes (CQEIST) for technical assistance with the thermogravimetry analysis.

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REFERENCES

(1) Liberman, A.; Mendez, N.; Trogler, W. C.; Kummel, A. C. Surf. Sci. Rep. 2014, 69, 132. (2) (a) Yokoi, T.; Karouji, T.; Ohta, S.; Kondo, J. N.; Tatsumi, T. Chem. Mater. 2010, 22, 3900. (b) Mihalcik, D. J.; Lin, W. ChemCatChem 2009, 1, 406. (3) Yang, Y.; Chengzhong, Y. Nanomedicine 2016, 12, 317. (4) (a) Caltagirone, C.; Bettoschi, A.; Garau, A.; Montis, R. Chem. Soc. Rev. 2015, 44, 4645. (b) Bae, S. W.; Tan, W.; Hong, J.-I. Chem. Commun. 2012, 48, 2270. 687


Functional Group Coverage and Conversion Quantification in

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