J. Phys. Chem. C 2007, 111, 1373-1377
1373
Spectroscopic and Morphological Investigation of an Optical pH Meter Based on a Porphyrin Monolayer Covalently Assembled on a Engineered Silica Surface Antonino Gulino,* Placido Mineo, and Ignazio Fragala` Dipartimento di Scienze Chimiche, UniVersita` di Catania and I.N.S.T.M. UdR of Catania, Viale Andrea Doria 6, 95125 Catania, Italy ReceiVed: October 4, 2006; In Final Form: NoVember 13, 2006
A covalently assembled monolayer of 5,10,15-{p-[9-methoxy-three(oxyethylene)]phenyl}-20-(p-hydroxyphenyl) porphyrin molecules was fabricated on silica substrates already functionalized with a covalent 4-ClCH2C6H4SiCl3 monolayer. The surface chemical characterization was carried out by angle resolved X-ray photoelectron measurements. Moreover, surface morphological characterizations were performed by atomic force microscopy measurements. Present results provide information on the grafting of porphyrin molecules.
1. Introduction Engineering of inorganic surfaces via covalent bonding of organic molecules represents an interesting approach to the synthesis of hybrid inorganic/organic nanomaterials.1-12 In this research field, as a part of an ongoing study regarding the engineering of silica substrates by covalent bonding of sensing molecules,13-18 we have recently described the surface evolution of transparent SiO2 substrates along the step by step grafting of appropriate coupling layer (CL) groups suited for successive anchoring of functional molecules.14 Moreover, we also described the first ever fabrication of an optical acid pH meter using a porphyrin monolayer covalently assembled on a molecularly engineered silica surface (called P-CAM).13 In these cases, transparent silica (quartz) substrates proved expedient for the covalent assembling of functional molecules. The synthesis procedure required first substrate grafting with bifunctional CL groups that can bond both to the substrate and to the following functional molecules. Therefore, the substrate was preliminarily silylated, with 4-ClCH2C6H4SiCl3 groups (CL) since Si-O bonds are environmentally stable for long periods. Then, porphyrin molecules were anchored, by appropriate chemical reactions, to the surface end groups (R) of the assembled CL molecules. The pH monitoring has important applications in many different fields ranging from the environmental applications (waste, river and seawaters, acid rain) to biological, medical, analytical, and industrial chemistry.19-20 Many pH-metering systems have already been proposed, mostly based on electrochemical measurements.19-20 Compared to these methods, optical techniques are particularly appealing due to the easy combination with optic fibers for remote sensing. Such methods require pH-dependent optical indicators and a suitable matrix materials to immobilize the indicator. In a previous work, we focused on the 5,10,15-{p-[9methoxy-three(oxyethylene)]phenyl}-20-(p-hydroxyphenyl) porphyrin (called P).13,15 This chromophore (see Figure 1a) shows a very high molar absorbance coefficient (1.42 × 105 in THF), a good affinity toward [H3O+], and some hydrophilic character due to the presence of the three 9-methoxy-three(oxyethylene) * Corresponding author. Tel: +39-095-7385067. Fax: +39-095-580138. E-mail:
[email protected].
Figure 1. Schematic drawing of 5,10,15-tri{p[9-methoxy-poly(ethyleneoxy)] phenyl}-20-(p-hydroxyphenyl)porphyrin (P).
groups covalently bounded in the peripheral positions of the porphyrin.13,15 A thorough characterization of porphyrin monolayers is crucial to understand the unique physicochemical properties of this system. There is, therefore, enough motivation to expand our preliminary results with a fully spectroscopic and morphological characterization of this P-CAM system. 2. Experimental Details The Porphyrin derivative used in the present investigation was synthesized, by condensation reactions between appropriate quantities of the sodium salt of 5,10,15,20-tetrakis(p-hydroxyphenyl)porphyrin and CH3-(OCH2-CH2)3-Cl. Moreover, it was purified and characterized as already reported.21 The pure sodium salt of P was obtained by reacting a tetrahydrofuran (THF) porphyrin solution with an equimolar amount of a methanol solution of sodium-tert-butoxide and removing in vacuum the tert-butylic alcohol formed during the reaction, THF and methanol. Three successive steps have been expedient for the fabrication of the P-CAM (Figure 1).13-15 Fused silica
10.1021/jp066523w CCC: $37.00 © 2007 American Chemical Society Published on Web 12/29/2006
1374 J. Phys. Chem. C, Vol. 111, No. 3, 2007 (quartz) substrates (i) were first cleaned with “piranha” solution (c H2SO4:30% H2O2, 70:30 v/v) at 80 °C for 1 h and then left to cool to room temperature. They were repeatedly rinsed with double-distilled water and immersed in a H2O:30% H2O2:NH3, 5:1:1 v/v/v mixture at room temperature for 40 min. Then, they were washed with double distilled water and dried under vacuum immediately before deposition of the coupling agent. All of the following sample manipulations have been performed in a glove box under inert atmosphere. Therefore, (ii) freshly cleaned substrates were immersed, at room temperature for 20 min, in a 1:100 (v/v) n-heptane solution of the chemisorptive siloxane, trichloro[4-(chloromethyl)phenyl] silane, to afford a monolayer of the coupling agent (CA).13-15 Then, they were washed with a copious amount of n-pentane and sonicated in acetone for 1 min to remove any physisorbed CA. Moreover, (iii) the silylated substrates were immersed in a 5 × 10-3 M THF solution of the present porphyrin sodium salt and heated up to 60 °C under stirring for 48 h.13,15 Finally, the substrates bearing the covalently bound porphyrin molecules were cooled to room temperature and sonicated with THF to remove any residual unreacted porphyrin. Molecular monolayer characterization of the P-CAM was carried out with X-ray photoelectron spectroscopy. This technique is ideal since it allows high vertical resolution and gives information on the bonding states of the grafted molecules by the analysis of angle-resolved measurements.14 In particular, angle resolved X-ray photoelectron spectra (AR-XPS) were measured at different takeoff angles, relative to the surface plane, (90°, 45°, 30°, 15°, and 5°) with a PHI 5600 Multi Technique System which offers a good control of the electron off-take angle (base pressure of the main chamber 2 × 10-10 Torr).14,22-23 The acceptance angle of the analyzer and the precision of the sample holder concerning the takeoff angle are (3° and (1°, respectively.14 The spectrometer is equipped with a dual anode X-ray source, a spherical capacitor analyzer (SCA) with a mean diameter of 279.4 mm, and an electrostatic lens system Omni Focus III. The nominal analyzer resolution was set to 400 meV. Samples were mounted on Mo stubs. Spectra were excited with Al KR radiation. The structure due to satellite radiation has been subtracted from the spectra before the data processing. The XPS peak intensities were obtained after Shirley background removal.24 Procedures to account for steady-state charging effect have been described elsewhere.22-23 Freshly prepared samples were quickly transferred from the glove box under N2 atmosphere to the XPS main chamber. Experimental uncertainties in binding energies lie within (0.5 eV. The spectra were deconvoluted by fitting the spectral profiles with a series of symmetrical Gaussian envelopes after subtraction of the background. The agreement factor, R ) [Σ(F0 - Fc)2/Σ - (F0)2]1/2, after minimization of the function Σ(F0 - Fc)2 converged to R values e0.04. Morphological characterizations were performed by atomic force microscopy (AFM) measurements with a Solver P47 NTD-MDT instrument in semicontact mode (resonance frequency 150 kHz). The noise level before and after each experiment was 0.01 nm. UV-visible measurements were performed using a UV-vis1601 Shimadzu spectrophotometer. Experimental uncertainty lies within (0.5 nm. 3. Results and Discussion 3.1. P-CAM Synthesis. Three successive steps achieved the synthesis of P-CAM on silica substrates (Figure 1):13,15 (i) Silica surfaces were first cleaned and hydroxylated; (ii) then, under
Gulino et al.
Figure 2. UV-vis spectra of the (a) 6 × 10-5 THF P solution, carried out using 0.1 cm quartz cuvettes and (b) P-CAM.
TABLE 1: XPS Atomic Concentration Analysis of the P-CAM
a
PTAa/°
Si 2p
O 1s
C 1s
N 1s
90 45 30 15 5
20.3 13.8 11.7 7.2 2.1
45.0 31.2 26.0 19.1 16.5
33.7 53.8 61.0 72.2 79.1
1.0 1.2 1.3 1.5 2.3
PTA ) photoelectron takeoff angle.
rigorously inert atmosphere, they were treated with the chemisorptive siloxane to afford a monolayer of the CL;14 (iii) finally, the appropriate functional molecule was covalently linked to the silylated substrates.13,15 3.2. UV-vis Measurements. The UV-vis spectrum of the resulting P-CAM (Figure 2b) shows high quality signals at 427.2, 519.9, 560.8, 596.1, and 654.7 nm. All of these bands find counterparts in those observed in the UV-vis spectrum of the P (6 × 10-5 M) THF solution (Figure 2a) that shows a characteristic sharp Soret band at 421.5 nm and satellite Q-bands at 517.7, 554.2, 596.2, and 655.5 nm. Using the Beer-Lambert law (A ) lc, where A is the absorbance and , l, and c are the extinction coefficient, the thickness of film, and the concentration of the film, respectively), one can calculate the surface coverage, dsurf ) A-1.25-26 The calculated density values of porphyrin molecules/cm2 in present P-CAM’s lies in the 1.6-1.9 × 1012 range, as expected for a densely packed covalently bound porphyrin monolayer. 3.3. AR-XPS Measurements. Table 1 collects atomic concentration data of prototypical P-CAM once the relevant atomic sensitivity factors have been accounted for.27 There is evidence of a monotonic decrease of both Si and O peaks while both C and N signals significantly increase. In particular, the Si 2p signal almost disappears on going from 90° (20.3%) to 5° (2.1%) photoelectron takeoff angles (Figure 3). This behavior is largely due to the falloff of the substrate component at 103.3 eV. Nevertheless, deconvolution of the 15° peak reveals the presence of an additional lower energy component at 102.0 eV that increases on going to 5°. Literature data suggest that this feature (102.0 B.E.) represents the fingerprint of Si coordinated by both oxide anions and an organic group.28-30 Therefore, we can confidently assign this peak to the siloxane framework.28-30
Investigation of an Optical pH Meter
Figure 3. Al KR excited XPS of P-CAM in the Si 2p energy region at different electron takeoff angles (90°, 45°, 30°, and 5°). Structures due to the satellite Al KR2 radiation have been subtracted from the spectra. The dashed line, superimposed to the experimental profile, refers to the sum of the Gaussian components.
Figure 4. Al KR excited XPS of P-CAM in the O 1s energy region at different electron takeoff angles (90°, 45°, 30°, and 5°). Structures due to the satellite Al KR2 radiation have been subtracted from the spectra. The dashed line, superimposed to the experimental profile, refers to the sum of the Gaussian components.
Similar conclusions can be arrived at by considering the O 1s signal. In fact, its intensity decreases on going from 90° (45.0%) to 5° (16.5%) (Table 1). Nevertheless, it remains about 1/3 of that obtained at 90°. Deconvolution of the grazing 5° profile (Figure 4) suggests that the 533.0 B.E. component, due to the silica substrate surface, sensibly decreases on decreasing of the photoelectron takeoff angle. Besides, a significant increase of the lower B.E. shoulder, at 532.6 eV, becomes evident. This feature is likely to be due to oxygens of polyether groups in the meso 5,10,15 porphyrin
J. Phys. Chem. C, Vol. 111, No. 3, 2007 1375
Figure 5. Al KR excited XPS of P-CAM in the C 1s energy region at different electron takeoff angles (90°, 45°, 30°, and 5°). Structures due to the satellite Al KR2 radiation have been subtracted from the spectra. The dashed line, superimposed to the experimental profile, refers to the sum of the Gaussian components.
positions. Moreover, the present B.E. value agrees well with literature data for polyethers.31-33 Photoelectron spectra of P-CAM in the C 1s region (Figure 5) show a peak at 284.8 eV that agrees well with literature data of porphyrin molecules.34-35 Interesting, we note an intensity increase of the high-energy component at 286.2 eV, on going from normal to 5° grazing photoelectron takeoff angles. This component, more evident in the 5° spectrum, evidences the aliphatic polyether carbon backbone and falls within literature data.31-33,36-37 Worthy of note, the whole carbon atomic concentration significantly increases from 33.7% at 90° to 79.1% at 5° emission angles (Table 1). Finally, the N 1s spectra (Figure 6) show a broad peak whose resolution becomes better at the most grazing emission angle here adopted. In fact, in the 5° spectrum, two clearly evident features at 400.1 and 398.1 eV were obtained, and the deconvolution became a straightforward matter. These features account for the two kinds of nitrogen in the free porphyrin base.34-35,38 The nitrogen atomic concentration also shows a monotonic increase upon decreasing the electron takeoff angle from 1.0% (at 90°), 1.2% (at 45°), 1.3 (at 30°), and 1.5 (at 15°) to 2.3% (at 5°), as expected for the upper layer nature of the signal (Table 1). In particular, both C and N XPS atomic concentrations increase by a 2.3 factor on going from 90° to 5° electron takeoff angles (79.1/33.7 ) 2.35; 2.3/1.0 ) 2.3). In this context, freshly prepared samples were quickly transferred from the glove box under N2 atmosphere to the XPS main chamber. Therefore, no other species are expected on the present P1-CAM surface, especially during XPS measurements (base pressure of the main chamber 2 × 10-10 Torr). Figure 7 shows the AR-XPS angular dependence of the IC/ Si I intensity ratios (IC and ISi are the total intensities of carbon and silicon, respectively) vs the photoelectron takeoff angle. The ratios exponentially decrease with the photoelectron takeoff angle θ, consistently with the presence of a carbonaceous
1376 J. Phys. Chem. C, Vol. 111, No. 3, 2007
Gulino et al.
Figure 8. AFM images of the P-CAM.
Figure 6. Al KR excited XPS of P-CAM in the N 1s energy region at different electron takeoff angles (90°, 45°, 30°, and 5°). Structures due to the satellite Al KR2 radiation have been subtracted from the spectra. The dashed line, superimposed to the experimental profile, refers to the sum of the Gaussian components.
Figure 7. XPS atomic concentrations vs the electron takeoff angle of the P-CAM.
overlayer on SiO2. The intensity of AR-XPS signals due to the silica substrate covered by an overlayer of thickness d and that due to the overlayer itself are, respectively27
ISi ) ISi ∞ exp
[
-d sen θ
λCSi2p
]
(1)
and
(
IC ) IC∞ 1 - exp
[
-d sen θ
λCCls
])
(2)
Combining eqs 1 and 2, the IC/ISi intensity ratio is
C
I ) ISi
(
[
-d sen θ -d ISi ∞ exp C λSi2p sen θ
IC∞ 1 - exp
[
λCCls
]
])
(3)
where λCSi2p and λCC1sare the mean free paths of Si 2p and C1s photoelectrons in a carbonaceous overlayer (4.15 and 3.6 nm,
C respectively).39-41 ISi ∞ and I∞ are the intensities of pure Si and C elements on the same instrument and setting. However, it is prudent to take the IC∞/ISi ∞ ratio as the ratio of the adopted Wagner sensitivity factors.27 Given this, eq 3 can be adopted to reproduce experimental data and provides an estimate of the thickness of the carbonaceous overlayer.27,39-41 The obtained d value of 29.75 ( 5 Å is strongly consistent with the presence of a porphyrin monolayer of the silica surface. The χ2 and R2 values of the fit are 1.06 and 0.996, respectively. In addition, the porphyrin size, whose geometry was optimized with an MM+ method, is within the d value when the experimental uncertainty is taken into account. In fact, the siloxane moiety size corresponds to 6.8 Å, the porphyrin core is about 18 Å, and the polyether group length is 12 Å. Therefore, the XPS d thickness suggests a monolayer of porphyrin molecules in the P-CAM. In summary, the XPS technique is very useful to study monolayers since provides chemical information that are unique and not available with many other spectroscopic techniques. Several authors already used AR-XPS results obtained using takeoff angles in the 90°-1° range for further quantitative calculations, e.g., to quantify the surface dopant coverage.42 Nevertheless, we are aware that for too small takeoff angles (
