J. Phys. Chem. C 2007, 111, 15217-15222
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Application of Infrared Interferometry for Quantitative Analysis of Chemical Groups Grafted onto the Internal Surface of Porous Silicon Nanostructures S. A. Alekseev,*,† V. Lysenko,‡ V. N. Zaitsev,† and D. Barbier‡ Chemistry Department, KieV National Taras SheVchenko UniVersity, 60 Vladimirskaya St., 01033 KieV, Ukraine, and Lyon Institute of Nanotechnologies, INL, CNRS UMR-5270, INSA de Lyon, 7 AVenue Jean Capelle, Baˆ timent Blaise Pascal, 69621 Villeurbanne Cedex, France ReceiVed: February 13, 2007; In Final Form: July 24, 2007
The internal surface of porous silicon (PS) nanostructures was chemically modified by octadecyl and carboxylic acid groups by applying the hydrosilylation reaction as well as by aminopropyl and vinyl groups applying the silanization reaction. Concentrations of the chemically grafted groups and thicknesses of grafted layers were determined by measurements of PS refractive index in the infrared spectral range. The Landau-LifshitzLooyenga effective media model was used to relate the measured refractive index values to a volume fraction and then to the concentration of the grafted groups. The described quantitative method was applied to determine the sensitivity limits of PS-based sensing devices.
1. Introduction Over the recent years, realizations of different types of sensors,1,2,3 chromatography chips,4 microreactors,5,6 fuel cells,7 and other devices using porous silicon (PS) nanostructures were reported. In fact, being potentially compatible with silicon-based technologies, PS exhibits many attractive structural properties, such as large internal surface area (up to 900 m2/cm3), adjustable porosity over a large range (up to 90%), and pore sizes (from 1 nm to 10 µm). Chemical functionalization of the enormous internal surface of porous silicon with organic fragments has found various applications.1-7 Two main approaches are usually used for chemical modification of the PS internal surface: (i) hydrosilylation, which consists of the reaction of alkenes with silane groups SiHx (with x ) 1, 2, or 3) of as-prepared PS under photochemical or thermal activation8,9 and (ii) mild oxidation of PS resulting in the formation of a hydroxylated SiO2 layer ready for the silane chemistry modifications, which are commonly applied to the silica-gel surfaces.10 Despite a huge amount of papers devoted to the chemical functionalization of PS, only few of them11,12 contain information on concentrations of organic groups grafted on the PS internal surface. Certainly, quantitative composition analysis of the grafted layers is a crucial point for the understanding of the surface-assigned properties of PS from both chemical and physical points of view. However, this analysis encounters some difficulties, such as low mass of PS samples, low contents of surface grafted groups, and the presence of mixtures of these groups in the case of PS samples, prepared via a multistage synthetic procedure. In contrast to conventional porous materials (silica-gel, for example), PS nanostructures can be produced as optically thin layers in which light interference occurs. Estimation of the PS refractive index from interference optical spectra and studying of the refractive index changes induced by chemical modifications in combination with the analytical model can allow * Corresponding author. † Kiev National Taras Shevchenko University. ‡ Lyon Institute of Nanotechnologies, INL.
quantitative analysis of the chemical groups grafted on the PS internal surface. In this paper, we present application of infrared interferometry to the quantitative analysis of the chemical groups grafted inside the PS nanostructures and, in particular, to the monitoring of the efficiency of PS oxidation and grafting with octadecyl groups, undecylenic acid groups, and silane molecules. In addition, the performed quantitative analysis allowed for the understanding of the morphological organization of groups grafted onto the PS internal surface. 2. Experimental Section 2.1. Porous Silicon Fabrication and Oxidation. Porous silicon was prepared by anodic etching of p+ type boron-doped (0.01-0.02 Ω cm) double-side polished (100)-oriented silicon wafers. The anodization was performed in a Teflon cell with an Au counter electrode and a backside Cu electrode. Silicon samples were etched in a solution containing 1:1 volume mixture of HF (49%) and ethanol applying 150 mA cm-2 anodic current density for 15 min. A permanent stirring of the etching solution was applied in order to evacuate hydrogen bubbles formed during the anodization process. Two seconds etch-stop intervals (2 s etch + 2 s stop) were introduced during the anodization in order to replenish the electrolyte in the depth of the nanopores and thus to avoid a porosity gradient along the layer thickness. At the end of the anodization process, formed PS layers were removed from the bulk silicon wafer by switching the anodization current to the electropolishing regime (1.27 A cm-2 current density pulse during 5 s) to form so-called free-standing PS layers. One free-standing PS sample referred to as PS-OX1 was oxidized at 523 K during 1 h in ambient air. Another one, referred to as PS-OX2, was first oxidized at 573 K during 1 h in a dry oxygen atmosphere, and then the oxide layer was densified13 by sample treatment at 973 K for 1 h in a dry nitrogen atmosphere. 2.2. Chemical Modification Procedures. The thermal hydrosilylation approach was used for the grafting of octadecyl and 10-carboxydecyl groups onto the PS internal surface. Freshly prepared PS samples were treated with neat 1-octadecene
10.1021/jp0712452 CCC: $37.00 © 2007 American Chemical Society Published on Web 10/03/2007
15218 J. Phys. Chem. C, Vol. 111, No. 42, 2007
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N2 - N1 ) 2dnPS[ν˜ 2 - ν˜ 1], where ν˜ 1,2 )
1 λmax1,2
(2)
In this work, two interference maxima (minima in absorbance scale) at spectral positions near 6500 cm-1 and 4000 cm-1 were taken for the nPS calculations (see Figure 1). Weak spectral dispersion of the nPS values was neglected. 3. Results and Discussion
Figure 1. Interference fringes on FTIR spectra of an initial PS free layer.
(sample PS-C18H37) or 10-undecylenic acid (sample PS(CH2)10CO2H) at 403 K for 16 h under a nitrogen atmosphere. A procedure of silanization was used to graft vinyl and aminopropyl groups on the PS surface. Oxidized PS samples (PS-OX1 and PS-OX2, respectively) were refluxed with 15% toluene solutions of 3-aminopropyltrimethoxysilane (sample PS-OX1-NH2) and vinyltrimethoxysilane (sample PS-OX2C2H3) for 16 h under a nitrogen atmosphere. After the reactions, the samples were successively rinsed with toluene, tetrahydrofuran, and hexane and then dried in ambient air. 2.3. Characterization Methods. Masses and thicknesses of the free-standing PS samples were measured using MettlerToledo balances with (0.01 mg precision and Mitutoyo IDC112B digital thickness meter with (0.5 µm precision, respectively. For the porosity determination by gravimetry, the density of the bulk silicon was taken as 2.33 g/cm3. Values of thickness and gravimetric porosity for all PS samples used in this work were found to be 56 ( 1 µm and 65 ( 2%, respectively. Fourier transform infrared (FTIR) spectra of the free-standing PS samples were recorded in the 370-7800 cm-1 spectral range in the transmittance mode at room temperature in ambient conditions using a Perkin-Elmer GSX-2 spectrometer. In order to be able to compare intensities of the spectral peaks, the area of the open space used for the detection of background transmitted signals was always kept the same as the area of the PS samples exposed to the infrared radiation. Interference of the infrared light on homogeneously thin PS layers is manifested on the FTIR spectra (see Figure 1) by sinusoidal fringes that can be clearly seen in the spectral region (>3500 cm-1), which is free from chemical bond vibration features. Positions of the interference maxima satisfy the following equation:
Nλmax ) 2dnPS
(1)
where nPS is an average refractive index of a PS layer, d is the layer’s thickness, λmax is the wavelength corresponding to an interference maximum, and N is an integer number corresponding to the maximum order. A difference between numbers N2 and N1, related to any two maxima of the interference fringes, measured thickness of the PS layer (d), and refractive index of PS (nPS), are interrelated due to the following expression, which can be derived from eq 1:
3.1. Qualitative Description of Chemical Groups Grafted on PS Internal Surface. The surface of a freshly prepared PS is covered by silane groups (SiHx with x ) 1, 2, or 3), which are detected due to their characteristic bands (νs(SiH2), 2137 cm-1; ν(SiH), 2114 cm-1; νas(SiH2), 2088 cm-1; δ(SiH2), 906 cm-1; ω(SiH2), 660 cm-1, and ω(SiH) at 625 cm-1) in the FTIR spectrum (see Figure 2).14,15 Oxidation of PS at 523 K resulted in the formation of O3SiH (ν(SiH), 2264 cm-1; νas(O-Si-O), 1200-1000 cm-1; ω(SiH), 875 cm-1), Si-OH (ν(O-H)isolated, 3745 cm-1; ν(O-H)H-bonded, wide band at 3400 cm-1) and probably {SiO4} surface units (see spectrum of PS-OX1 sample in Figure 2). However, the bands of νs(O-Si-O) at 810 cm-1 and δ(SiO2) and 470 cm-1, which are typical for bulk SiO2,16 are not observed in the spectrum of PS-OX1; hence, formation of a dense and continuous SiO2 layer cannot be stated for this sample. The opposite is true for the strongly oxidized PS-OX2 sample, as the bands in its FTIR spectrum (Figure 2) are identical to those of amorphous silica-gel. Hydrosilylation reaction was used for the preparation of PSC18H37 and PS-(CH2)10CO2H samples (Scheme 1). SCHEME 1
Chemical composition of the derived materials was confirmed by the presence of characteristic bands,17 corresponding to chemically grafted groups, in the FTIR spectra (Figure 3). We have detected bands of νs(CH2) at 2853 cm-1, νas(CH2) at 2925 cm-1, and δ(CH2) at 1467 cm-1 for both samples; νas(CH3) at 2962 cm-1 for PS-C18H37; and ν(CdO) at 1715 cm-1 for the PS-(CH2)10CO2H sample. Absence of the ν(CdC) band at 1645 cm-1 in the spectra of both samples signifies the total absence of physically adsorbed molecules of the organic reagents, which bear double bonds. A significant decrease of the intensities of the SiHx bands in the spectra of hydrosilylated samples compared to the spectrum of the initial PS indicates partial consumption of the SiHx groups in the reaction course (Scheme 1). The reaction of silanization, used for the preparation of the PS-OX1-NH2 and PS-OX2-C2H3 samples, consisted of the interaction of surface silanol groups of the oxidized PS samples with the active group of silane molecules due to Scheme 2. SCHEME 2
As also seen in the case of hydrosilylation, the appearance of characteristic bands of grafted groups in the FTIR spectra (Figure 4) certifies that the reaction was successful. There are
Application of Infrared Interferometry
Figure 2. FTIR spectra of an as-prepared free PS layer and two oxidized PS samples: PS-OX1 and PS-OX2.
Figure 3. FTIR spectra of an as-prepared free PS layer and of two PS samples modified via the reactions of hydrosilylation: PS-C18H37 and PS-(CH2)10CO2H.
J. Phys. Chem. C, Vol. 111, No. 42, 2007 15219 spectrum of the PS-OX2-C2H3 sample. Interaction of vinyltrimethoxysilane with the oxidized internal surface of PS-OX2 results in the consumption of isolated silanol groups, which are known to be the most active species in the silanization reaction.10 Their band ν(O-H)isolated at 3750 cm-1, which was prominent in the spectrum of PS-OX2 (Figure 4), nearly disappeared in the spectrum of the PS-OX2-C2H3 sample. 3.2. Refractive Index and Porosity of As-Prepared PS Samples. There are several effective media approximations (EMA) allowing estimation of the effective dielectric function and thus the refractive index of the multicomponent nanodispersed systems and the PS in particular.15 The most recent and general EMA for two-phase porous systems was established by Bergman.19 However, besides the parameter of porosity, the Bergman representation involves a function called spectral density, which is characteristic for the microtopology of the sample. Unfortunately, exact calculations of the spectral density in the case of the complex microtopology of PS are extremely difficult, especially for such a multicomponent system as chemically modified PS. The most prominent Bruggeman20 and Maxwell Garnett21 EMAs are in principle the particular cases of Bergman EMAs for the geometries “mixture of the spherical particles” and “spherical particles in the media”, respectively. Despite the fact that the Bruggeman EMA is one of the most frequently and successfully used approximation for the determination of PS porosity via optical measurements, the real microtopology of PS is only vaguely consistent with the one laying at the ground of this model. Because of our opinion, a model, named Looyenga-LandauLifshitz (3L) EMA,22,23 seems to be more reliable for the PS and chemically modified PS samples. This model is based on the conditions of sample optical homogeneity and relatively small differences between dielectric functions of the components; the influence of the sample microtopology is neglected. In fact, as it is presented in ref 24, the results of the 3L EMA calculations of the PS refractive index correlate well with the results of the most frequently used Bruggeman EMA. Unlike all other models considered, the 3L model can be easily applied to the systems, containing as many components as one may wish for without any computing complications. According to the 3L model, the average effective refractive index of a complex dispersed system can be expressed as follows:
n2/3 a )
Figure 4. FTIR spectra of two oxidized PS samples chemically modified via the reactions of silanization: PS-OX1-NH2 and PSOX2-C2H3.
weak bands of ν(NH2) at 3368 and 3290 cm-1, bands of δ(NH2) at 1570 and 1485 cm-1, and ν(CH2) at 2932 and 2883 cm-1 in the spectrum of the PS-OX1-NH2 sample. The presence of two δ(NH2) bands indicates the presence of hydrogen bonding between -NH2 and the silanol groups.18 Bands of ν(CH) at 3070, 3030, 2992, and 2965 cm-1 (-CHd CH2 group); 2851 cm-1 (residual -OCH3 group); ν(CdC) at 1605 cm-1; and δ(dCH2) at 1412 cm-1 are found in the
∑i fin2/3 i
(3)
where ni are the refractive indexes and fi are the volume fractions of different components constituting a given nanodispersed system. Additionally, the condition ∑ fi ) 1 should be i satisfied. In the case of chemically nonmodified (as-prepared) PS layers, only two phases constitute the PS nanostructures: (i) solid-state silicon nanocrystallites with a refractive index nSi ) 3.4 and (ii) air, which fills the empty nanopores, with nm ) 1. Therefore, according to the 3L model, the PS refractive index nPS can be expressed as follows: 2/3 2/3 n2/3 PS ) fSinSi + f0nm
(4)
where f0 is the porosity of the as-prepared PS layer and fSi ) 1 - f0 is the volume fraction occupied by the nanocrystallites. For the PS sample with thickness d ) 55 µm, a spectrum of which is presented in Figure 1, one can easily obtain values of nPS ) 1.748 (eq 2) and consequently f0 ) 64.2% (eq 4), which is in good agreement with the value obtained by gravimetry
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Figure 5. Schematic representation of oxidation and grafting of organic groups onto the internal surface of PS nanostructures.
(65.2%). The difference between the values of porosity, found by these two methods for all studied initial PS samples does not exceed (2.5%. 3.3. Interferometric Analysis of the Chemically Modified PS Samples. Oxidation of PS and grafting of organic groups on its internal surface influences the refractive index of the PS layers in different ways. Indeed, as it is illustrated in Figure 5, oxidation of the PS internal surface leads to the partial transformation of the silicon nanocrystallites with high refractive index (nSi ≈ 3.4) into a silicon oxide layer with a lower refractive index value. As for hydrosilylation and silanization, it results in the partial filling of initially empty PS nanopores (with refractive index nm ) 1) by a layer of chemically attached organic groups with refractive indexes nR > 1. Therefore, the overall refractive index of PS decreases upon oxidation and increases upon the grafting of organic groups. To estimate volume fractions and concentrations (CL) of the grafted groups in chemically modified PS samples from refractive index measurements, the values of refractive indexes and densities of the grafted surface layers were assumed to be equal to those of hydrocarbons in the liquid phase for alkyl layers (nR ) 1.43 and FR ) 0.77 g/cm3) to those of quartz glass for the SiO2 layer (nSiO2 ) 1.5 and FSiO2 ) 2.2 g/cm3) and to those of liquid silanes for the silanization-grafted layers (nR ) 1.5, and FR ) 1.0 g/cm3). Nonoxidized PS, which contains organic groups, for example grafted by the hydrosilylation procedure (Scheme 1), can be considered as a three-component nanodispersed system, consisting of silicon, an organic layer, and remaining media that fills the nanopores. According to the 3L effective media model given by eq 3, the refractive index of such a system can be expressed as follows: 2/3 2/3 2/3 n2/3 PS ) fSinSi + fRnR + fmnm
(5)
where fR and fm are the volume fractions occupied by the grafted organic groups and by the remaining media, respectively. The volume fractions of all the three phases must satisfy the following equations:
fSi + fR + fm ) 1 and fR + fm ) f0
2/3 2/3 2/3 nPS-OX ) f/Sin2/3 and Si + fSiO2nSiO2 + fmnm
f/Si + fSiO2 + fm ) 1 (8) To calculate the volume fractions of Si nanocrystallites (f/Si), SiO2 phase formed after oxidation (fSiO2), and nanopores of the oxidized PS layer (fm), the volume expansion of the partially oxidized Si nanocrystallites should be taken in account. The volume fraction of the reacted silicon, fSi(R), is related to the volume fraction of the formed SiO2 phase due to the equation:
fSi(R) )
MSiFSiO2 f ) 0.44fSiO2 MSiO2FSi SiO2
(9)
The volume fraction of the remaining (nonoxidized) silicon, f/Si (in eq 8), can be expressed as: f/Si ) fSi - fSi(R), where fSi is the volume fraction of silicon nanocrystallites in the initial asprepared (before oxidation) PS sample. The volume fraction of the nanopores (porosity), fm, in the oxidized sample is equal to fm ) f0 - (fSiO2 - fSi(R)), where f0 is the porosity of the asprepared PS sample. Taking into account eqs 4, 8, and 9, the following relation can be derived for the calculation of the difference between the refractive indexes of PS before (nPS0) and after oxidation (nPS-OX): 2/3 2/3 2/3 2/3 n2/3 PS0 - nPS-OX ) fSiO2(0.44nSi - nSiO2 + 0.56nm )
(10)
(6)
where f0 is the porosity of the initial as-prepared PS layer before the grafting. The difference between the refractive indexes of PS after (nPSa) and before (nPS0, see eq 4) the grafting of an organic layer can be expressed as follows: 2/3 2/3 2/3 n2/3 PSa - nPS0 ) fR(nR - nm )
species. For example, we used the refractive indexes of oxidized PS layers as nPS0 and the refractive indexes of PS after silanization as nPSa in order to calculate fR for the PS-OX1NH2 and PS-OX2-C2H3 samples. To avoid any influence of thickness and porosity deviations of different samples, we always compared the refractive indexes of one particular sample before and after each modification step. Oxidized PS can be considered as a three-component nanodispersed system, consisting of silicon, SiO2, and the remaining media inside the nanopores. Its refractive index can be expressed as follows:
(7)
For example, hydrosilylation grafting of -C18H37 groups on the PS sample with nPS0 ) 1.748, considered in the previous section, gave us a sample PS-C18H37 with nPSa ) 1.882. A value of fR ) 0.272 was derived from eq 7. The same eq 7 can be used for the determination of fR after any elementary step of the multistage chemical modification of the PS surface if this modification step consists of the substitution of the media inside the pores by grafted or adsorbed
3.4. Quantitative Parameters of Surface Layers of the Chemically Modified PS Samples. Presented in the previous section, equations were used for the calculation of the volume fractions (fR) of grafted organic layers or oxide layers for all studied samples (Table 1). To achieve better understanding of the grafted layers peculiarities, their thicknesses (h) and surface groups concentrations related to the total volume of PS sample (CL) were estimated on the basis of the fR values. Thickness h was estimated due to the relationship h ) fR/S, where S is the value of the internal surface area of the as-prepared PS samples, taken to equal 200 m2/cm3 due to the literature data25 for the PS formed on p+ type Si substrates under anodization conditions similar to those applied in the presented work. Thicknesses of monomolecular layers (hmono) were estimated, considering the tetrahedral geometry of chemical bonds in C and Si atoms and the lengths of single C-C (0.154 nm) and Si-O (0.162 nm) bonds.
Application of Infrared Interferometry
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TABLE 1: Volume Fractions (fR), Groups Concentrations (CL), Thicknesses of Grafted Layers (h), and Theoretical Values of Thicknesses in the Case of Monomolecular Layer (hmono)a sample
fR, %
h, nm
hmono, nm
CL(n), mmol/cm3
CL(w), mmol/cm3
PS-C18H37 PS-C10H20CO2H PS-OX1 PS-OX1-NH2 PS-OX2 PS-OX2-C2H3
27.1 13.4 5.3 22.1 48.3 5.3
1.36 0.67 0.26 1.10 2.41 0.27
2.26 1.38 0.26 0.60 0.26 0.34
0.83 0.56 1.93 1.92 17.69 0.63
1.07 1.74 2.62 14.83
a The estimations of CL were performed from interferometric CL(n) and gravimetric CL(w) measurements.
The concentrations of the grafted groups were obtained as CL(n) ) (fRFR)/MR, where MR is a formula weight of the grafted fragment. We have used weights of the unsaturated molecule for hydrosilylation, of SiO2 for oxidation and of a combination {(CH3O)3Si-R - 2 CH3OH} for silanization, assuming that the reaction proceeds due to the stoichiometry presented in Scheme 2. With the use of the same values of the grafted fragment formula weights, the values of concentrations (CL(w)) were calculated using of the gains of the sample masses upon the modification. Values of CL, obtained by interferometry and gravimetry, are in reasonable agreement (Table 1). Hence, the sequence of calculations, proposed for the handling of PS refractive index data, appears to be acceptable at least for the estimation of chemically grafted groups concentration in PS. Analysis of the derived concentration values can shed light on the grafted layers morphology. For PS-C18H37 and PS-C10H20CO2H samples, the real thicknesses of grafted layers appears to be approximately 2 times smaller than the estimated monolayer thicknesses (i.e, lengths of grafted hydrocarbon chains). Thus, we can say that the grafted layers have a disordered liquidlike morphology rather than a self-organized monolayer structure. This conclusion can be of interest for the applications of chemically modified PS nanostructures in chromatography microdevices. Indeed, as it was recently shown,26 disordered alkyl layers demonstrate better chromatographic performance than dense monolayers due to better accessibility of the alkyl chains to the molecules in a solution. The concentration of the -C18H37 groups measured by gravimetry is found to be slightly higher than that deduced from the interferometric measurements. Partial oxidation of the PSC18H37 samples (note the existence of a well visible ν(Si-O) vibration band at 1100 cm-1 in the spectrum of PS-C18H37, Figure 3) is one of the possible reasons for it. Indeed, the partial oxidation of the PS layer, resulting in an increase of its mass, leads to an overestimated concentration value of the alkyl species given by the gravimetry, while the oxidation-induced decrease of the nPS value is responsible for the underestimated concentration value obtained by the interferometric method. Therefore, the real concentration value of the -C18H37 groups should be in the range of 0.83-1.07 mmol/cm3. The thickness of the SiO2 layer in the oxidized PS samples corresponds to one monolayer in the PS-OX1 sample and to nine monolayers in the PS-OX2 sample. These estimated values allow us to regard the oxide layer formed on the PS-OX1 sample as a sub-oxide rather than a stoichiometric SiO2 phase. Indeed, as it has already been shown in Figure 2, the PS-OX1 sample contains a large number of O3Si-H groups and the vibration bands characteristic for a dense SiO2 structure at 810
and 470 cm-1 are absent. Even in the case of such a thin SiO2 surface layer, the interferometric data worked up using the 3L model gave us a concentration value, which is quite close to the result of gravimetric measurements. The concentration value of the grafted aminopropyl groups is pretty large, and the thickness of the grafted aminopropylsilane layer on the PS internal surface is much larger than the one of the monolayer. It indicates the formation of a so-called polysiloxane layer 10 as depicted in Scheme 3. Such a layer can be formed only if water molecules were present in the reaction mixture during the silanization stage. We suppose that the water was initially adsorbed on the surface of the hydrophilic PS-OX1 sample. SCHEME 3
Similar to the PS-C18H37 sample, the concentration of the aminopropyl groups in the PS-OX1-NH2 sample, derived from gravimetric measurements, appears to be larger than that estimated by the interferometric approach. Therefore, we assume that additional oxidation of the initially oxidized PS sample takes place during the silanization step. Indeed, the presence of the basic -NH2 groups accelerates additional oxide formation during the silanization step in the presence of water as the oxide layer in the initial PS-OX1 sample is not thick enough to prevent the PS surface from further oxidation. Indeed, as it was recently reported by our research group,27 an efficient oxidation of PS layers by water takes place even in extremely weak basic environments. Moreover, interaction of the freshly prepared PS with basic 3-aminopropyltrimethoxysilane in the presence of water was shown to result in PS oxidation as well as silane grafting.28 The thickness of the silane layer grafted onto the PS-OX2 sample is comparable to the thickness of the corresponding monolayer (Table 1). Thus, monomeric grafting of the silane (Scheme 3) occurs predominantly in this case. Increased hydrophobicity of the PS-OX2 sample compared to the PSOX1 one, which can be easily deduced from the intensities of the adsorbed water/H-bonded silanol band at 3400 cm-1 in their FTIR spectra as well as the absence of the basic -NH2 group (a catalyst of silane polycondensation,10) in the viniltrimethoxysilane molecule are the main reasons leading to the inhibition of the polysiloxane layer formation on the PS-OX2-C2H3 sample. 3.5. Estimation of the Detection Limits for the PS-Based Interferometric Sensing Devices. The effect of the PS refractive index change induced by adsorption of various molecules on the PS internal surface is widely applied in the design of PS-based interferometric sensing devices, especially biosensors and sensors based on capillary condensation phenomenon.29 Analytical signal of a PS-based optical device is in fact a shift of the reflectance (or transmittance) peak, due to the refractive index change induced by the analyte adsorption. Presented above, eq 7 interrelates the values of the refractive index change 2/3 2/3 - nPS0 ) and the amount of adsorbed under adsorption (nPSa analyte. This equation can be used not only for the free layers
15222 J. Phys. Chem. C, Vol. 111, No. 42, 2007 of PS but also for all PS-based devices with more complex nanostucture geometries, such as Bragg mirrors, Fabry-Perot cavities, and Rugate optical filters, which are typically used for sensing purposes.30-33 Thus a detection limit of the PS-based optical sensor depends on the accuracy of nPS measurements. For example, in the case of the PS free layer, the refractive index is determined from eq 2. Considering a standard FTIR resolution equal to 1 cm-1, a minimal detectable value of 2/3 2/3 - nPS0 ) can be estimated to be equal to 10-3, which (nPSa corresponds quite well to the known accuracy for determination of nPS by the PS-based optical devices with complex geometry or by other possible methods of refractive index measurements. The difference between the refractive indexes of an analyte 2/3 molecule and of a medium filling the nanopores (n2/3 R - nm ) is equal to 0.27 for organic compounds (nR ) 1.43) in air (nm ) 1) and to 0.06 for the organic compounds in water (nm ) 1.33). Thus, the detection limit of the volume fraction occupied by organic groups adsorbed onto the PS internal surface, fR, is equal to 0.37% in air and to 1.67% in water (see eq 7). In first-order approximation, it does not depend on the porosity, the surface area, or the thickness of the PS layer. The minimal detectable value of fR estimated for the measurements in air is perfectly acceptable for the detection of capillary condensed vapors. However, the minimal possible fR value which can be detected in water solutions can be hardly accepted for design of a sensor based on specific adsorption phenomenon. Indeed, it means that for an organic compound with a molar mass of 100 g/mol, the required adsorption in the PS sensor layer has to reach 150 µmol/ cm3 for it to be detectable in the water solution. 5. Conclusions Fourier transform infrared (FTIR) spectroscopy was successfully applied for simultaneous qualitative and quantitative analysis of chemical groups grafted onto the internal surface of the PS nanostructures. First of all, FTIR spectra of free-standing as-prepared and chemically modified PS layers were recorded in the 370-4000 cm-1 spectral range for qualitative control of chemical bonds present at the PS internal surface before and after specific chemical modifications. Interferometric measurements of the PS refractive index in the high energy infrared spectral range (4000-7800 cm-1) by the same FTIR setup, worked up using a Landau-Lifshitz-Looyenga effective media model, allow one to find the values of grafted molecules concentrations, which are sufficiently close to the values obtained by the gravimetric method. Performed quantitative analysis shed the light on the morphological organization of molecules grafted onto the PS internal surface. Application and limits of the described quantitative method for sensing devices based on PS nanostructures were discussed. Acknowledgment. This work was partially supported by NATO “Science for Peace” Grant SfP-981786 “Development of a novel sensing technique based on nanomechanics for rapid detection of bioagents” (Prof. V. N. Zaitsev) and individual President of Ukraine grant for young scientists “Porous silicon with covalently grafted ion-exchange and complexing groups, the novel material for sensor technologies” (Dr. S. A. Alekseev). FTIR measurements reported in this paper were performed with the use of facilities of the CECOMO measurement center from Lyon Claude Bernard University. The authors are grateful to Prof. B. Champagnon and to all staff members of the center for their technical support. Dr. S. A. Alekseev acknowledges
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