Solvophobic versus Electrostatic Interactions Drive Spontaneous

Article pubs.acs.org/JPCC

Solvophobic versus Electrostatic Interactions Drive Spontaneous Adsorption of Porphyrins onto Inorganic Surfaces: A Full Noncovalent Approach Alessandro D’Urso,‡ Alessandro Di Mauro,† Alessandra Cunsolo,‡ Roberto Purrello,‡ and Maria Elena Fragalà*,† †

Dipartimento di Scienze Chimiche, Università degli Studi di Catania and INSTM UdR di Catania, v.le A. Doria 6, 95125 Catania, Italy ‡ Dipartimento di Scienze Chimiche, Università degli Studi di Catania, v.le A. Doria 6, 95125 Catania, Italy S Supporting Information *

ABSTRACT: The successful transferring of a porphyrin complex assembly from a solution to a solid surface is important for an economical development of functional materials, biomaterials, and sensing devices. The understanding of the mechanisms and factors that drive spontaneous and stable deposition, in water, of porphyrins (and their complex species) onto inorganic surfaces paves the way for a straightforward and environmentally friendly noncovalent functionalization of solid surfaces. Here, we show that surface charge considerations need to be carefully considered if watersoluble porphyrin derivatives have to be successfully immobilized onto zinc oxide (ZnO) layers deposited on glass. In particular, it will be demonstrated that the electrostatics of the glass support has a central role in driving the layering of charged (anionic and cationic) porphyrin derivatives. Finally, our results underline the robustness and versatility of the noncovalently driven depositionin terms of both reproducibility and stability of the porphyrin assembliesand shed light on the crucial role played by dispersion interactions, which (in our experimental conditions) prevent the more specific covalent interactions.

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INTRODUCTION Spontaneous adsorption of molecules from a solution onto a solid surface is a common phenomenon, but its rationalization and control is extremely complex since it is driven by different weak interactions, including van der Waals, hydrophobic, and electrostatic forces.1,2 This aspect is particularly critical in water, where dispersive interactions are dominant. Therefore, to firmly attach water-soluble systems on a solid surface, an overall energy gain, able to overcome the unfavorable conditions experienced during their transfer from liquid to solid phase, needs to be accomplished. In past decades, spontaneous immobilization of molecules and biomolecules onto inorganic surfaces has been widely investigated,2−4since it represents the least technically challenging immobilization procedure, despite the difficulties of controlling a process that is affected by many variables, such as pH, temperature, ionic strength, nature and composition of adsorbed species, adsorbing surface, and solvent. Noteworthy, because oxide surfaces develop a surface charge in water (depending on the pH),5 electrostatic interactions play an important role when also absorbing molecules bear a charge. In this perspective, it is possible to exploit these spontaneous processes to obtain organic−inorganic hybrid nanomaterials. © 2013 American Chemical Society

Under this light, porphyrins are extremely attractive macrocyclic compounds because they can be easily derivatized with different functional groups with fixed or tunable charge, which renders them water-soluble but also allows for tuning their hydrophobicity. In addition, these macrocycles are characterized by an extensively conjugated two-dimensional π system, leading to unique photochemical and photophysical properties. Finally, the four nitrogen atoms in the core are ready to accommodate various metal ions, which will conjugate their coordination features with porphyrin chemistry. All of these properties make them unique molecules endowed with superior molecular recognition and sensing properties.6,7 Inorganic surfaces functionalized with porphyrins have recently gathered interest for their potential applications as light-harvesting devices,8−10 chemical sensors,11,12 and many other biological and chemical applications.13,14 In particular, porphyrins containing carboxylic or phosphonic functionalities have been often referred to as the best choice to covalently functionalize wide-band-gap semiconductor oxides, such as Received: June 4, 2013 Revised: July 19, 2013 Published: July 29, 2013 17659

dx.doi.org/10.1021/jp405514m | J. Phys. Chem. C 2013, 117, 17659−17665

The Journal of Physical Chemistry C

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Scheme 1. Solid Surfaces and Porphyrin Derivatives Used to Fabricate Hybrid Inorganic−Organic Materials

and ε412.5 = 3.2 × 105 M−1 cm−1 for DPPS (Figures S2−S4, Supporting Information). A 25 mg portion of poly-L-lysine (PLL, Sigma Aldrich Dp 465) was first dissolved in 5 mL of water and further diluted to obtain a working concentration of 1 mM. Surface Functionalization. The investigated oxide surfaces and porphyrin derivatives have been summarized in Scheme 1. The binding of porphyrins onto bare glass (1) and ZnO/ glass substrates (2) was accomplished by dip and wait (30−120 min) into diluted aqueous solutions (1 μM) of anionic (H2TPPS, DPPS) and cationic (H2T4) porphyrin derivatives. After dipping, samples were rinsed with water to remove unbound molecules. The amount of immobilized porphyrins was estimated by UV−vis spectroscopy, by direct analyses of the glass substrates or by desorption experiments in acid (samples were immersed in 2 mL of 0.12 M HCl for 1 h). Three to five samples were tested to confirm the process repeatability. Glass/polylysine (PLL) substrates (3) were prepared by dip and wait (ranging from 24 to 1 h) using glass slides (1) in a PLL solution (1 mM) at different pHs (ranging from 5 to 10). The functionalized surfaces (3) were treated with cationic and anionic porphyrin derivatives and optically analyzed by UV−vis spectroscopy. UV−vis Measurements. A UV−vis spectrophotometer (JASCO V-635) was used to characterize the optical properties of uncoated and porphyrin-functionalized substrates as well as the porphyrin desorbed solutions. Wavelength scans were collected from 300 to 700 nm at room temperature using a wavelength step size of 0.5 nm.

ZnO or TiO2.9,15−18 However, a noncovalent approach, able to provide stable sensitive layers immobilized onto inorganic surfaces, is extremely attractive as a facile strategy to be implemented in the field of solid-state sensors.19 In this perspective, we aim to investigate the capability of water-soluble porphyrin derivatives to promptly interact with inorganic (oxide) surfaces by electrostatic interactions. In fact, remarkably robust porphyrin films have been fabricated using the Langmuir−Blodgett (L-B) approach20,21 or layer-by-layer deposition.22,23 Awarded that the main forces that govern spontaneous adsorption at the surface−solution interface are hydrophobic and electrostatic interactions, herein, we envisage the critical role of the substrate (glass) surface charge to govern the outcome of spontaneous adsorption of anionic and cationic porphyrin derivatives. The presented noncovalent functionalization is confirmed effective in terms of process reproducibility and control as well as robustness of the assembled layers, thus resulting in a promising route for future development in nanotechnology and material science. Noteworthy, the use of water, as a process medium, makes this approach easy-tohandle and eco-friendly. The obtained results represent an important step toward the transfer of supramolecular chemistry versatility24 from aqueous solutions to the solid state and development of stereoselective porphyrin-based sensors.

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EXPERIMENTAL SECTION Sample Preparation. Quartz slides (1 mm thick, Helios Italquartz) have been used after cleaning to remove surface contaminants. ZnO depositions have been performed in a reduced pressure (5 Torr) chemical vapor deposition (CVD) hot-wall reactor in an Ar/O2 atmosphere at 600 °C, for an overall process time of 1 h.25 ZnO films (Figure S1, Supporting Information) have been deposited on both of the glass supports’ sides (ZnO/glass/ZnO). Reagents. Tetracationic meso-tetrakis(4-N-methylpyridyl)porphyrin (H2T4), tetra-anionic meso-tetrakis(4-sulfonatophenyl)porphyrin (H2TPPS), and dianionic 5,10-bis(4-sulfonatophenyl)-15,20-diphenylporphyrin (DPPS) were dissolved in Milli-Q filtered water to obtain aqueous solutions at desired concentrations (1 μM). The concentration of the porphyrin derivatives was determined from the UV−vis absorption spectrum using the extinction coefficient ε423 = 2.26 × 105 M−1 cm−1 for H2T4, ε411 = 5.33 × 105 M−1 cm−1for H2TPPS,

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RESULTS Porphyrin Tetra Derivatives Adsorption on Glass: Effect of ZnO Deposition. The isoelectric point (IEP) of metal oxides in aqueous solution plays a fundamental role to assist spontaneous immobilization of molecules and biomolecules bearing an opposite charge.26,27 Silica surfaces possess a low isoelectric point (IEP ∼ 2) that, in turn, implies that they are negatively charged at neutral pH, thus promoting the spontaneous adsorption of positively charged species.28 In good agreement with literature data,29 UV−vis spectra of (1) dipped in H2T4 aqueous solutions show a well evident band at λMAX = 428.5 nm (Figure 1, red line). The red shift (∼7 nm) of the Soret band associated with the H2T4 immobilized on glass surface component with respect to the Soret position recorded 17660



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Figure 2. UV−vis spectra (Soret region) of H2T4 (red line; λMAX = 435 nm) and H2TPPS (black line) desorbed from (2).

Figure 1. UV−vis spectra (Soret region) of H2T4 (red line; λMAX = 428.5 nm) and H2TPPS (black line) treated glass substrates (1).

should favor porphyrin absorption (for this reason, TPCC is commonly used to functionalize TiO2 and ZnO surfaces for photovoltaic application).8−10 The latter experiment strongly suggests that also the very favorable energy, due to the covalent bond between the ZnO surface and the carboxylate groups, is not enough to overcome the larger unfavorable term due to the electrostatic repulsion between anionic porphyrins and the surface. These results contradict our preliminary assumption about the supposedly positive surface charge of (2) and show that, on the contrary, ZnO deposition is not effective in reversing the glass surface potential, which is still quite negative. Yet, a small shift toward a less negative value is revealed by the lower amount of H2T4 immobilized on (2) with respect to that measured on (1), as shown in Figure 3.

in solution (λMAX = 421.5 nm; Figure S2, Supporting Information) and the related broadening point to a possible porphyrin side-by-side interaction30 or, according to the more recent literature, to the flattening of the meso substituent with respect to the plane of the porphyrin ring.31 On the contrary, anionic derivatives (H2TPPS) are strongly repelled by (1), as confirmed by the lack of any feature in the associated absorption spectrum, shown for comparison in Figure 1 (black line). Opposite to glass, ZnO surfaces bear an IEP > 95 in water, and accordingly, several references report about spontaneous adsorption, at neutral pH, of negatively charged species onto ZnO nanopowders and nanoparticles.32 On the basis of this evidence, we expected a marked affinity of anionic porphyrin derivatives onto the ZnO/glass substrate (2), which presents a complete ZnO coverage, as confirmed by XPS analysis (Table 1, Supporting Information). Following the same experimental procedure used for (1), samples (2) were first dipped in H2T4 or H2TPPS solutions (1 μM) for 2 h and then rinsed and dried. The reduced transparency of (2), due to ZnO deposition, makes the quantification of adsorbed porphyrin directly from the UV−vis spectra of the solid substrate more difficult. Therefore, desorption experiments in acidic solution (pH < 1) were performed (glass charge is reversed at pH < 2,5 and, in addition, it is well-established that ZnO is dissolved in acid solution33) in order to give a rough estimation of the amount of bound porphyrins. Surprisingly, our experimental evidence disagrees with the aforementioned assumption of an increased affinity of (2) through the anionic derivative, pointing again to a successful interaction of (2) only with the tetracationic H2T4. The UV−vis spectra of desorbed solutions show, in fact, a well-detectable H2T4 absorption band in the Soret region (λMAX = 435 nm; Figure 2), whereas no corresponding H2TPPS signal is observed (Figure 2, black line).43 It is important to stress the reproducibility of the experimental data shown in Figures 1 and 2, which has been further confirmed also in other experimental conditions (i.e., changing porphyrin concentration and substrate dipping time). What is more, similar “unexpected” results have been obtained by performing the same experiments with the tetracarboxylated (TPCC) derivative (not shown since no UV−vis signal has been detected) for which the carboxylate binding to zinc oxide

Figure 3. UV−vis spectra (Soret region) of H2T4 desorbed from (1) (red line; λMAX = 426.6 nm) and from (2) (black line; λMAX = 435 nm).

An interesting result is obtained by using the dicationic 5,10bis(4-sulfonatophenyl)-15,20-diphenylporphyrin (DPPS, 1 μM), bearing only two anionic sulfonate groups in the cis position. This derivative is slightly adsorbed on (2), but not on (1), as is well-evident from the UV−vis spectra of the desorbed solutions (Figure 4).44 17661



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Scheme 2. Schematic Drawing of Glass/PLL (3) Sample Preparation Procedure

Figure 4. UV−vis spectra (Soret region) of DPPS desorbed from (1) (black line) and from (2) (red line; λMAX = 433.5 nm).

Since DPPS bears two negative charges, according to our experimental results, its absorption on the anionic surface of (2) cannot be driven from electrostatics. Consequently, we hypothesize that the main forces driving adsorption at the (2) surface are solvophobic in nature. It is worth recalling, at this point, that porphyrins are water-soluble but still mainly hydrophobic molecules. Therefore, we can speculate that a spontaneous adsorption of DPPS on the investigated solid surfaces could be successfully achieved only if a crucial balance between solvophobic and electrostatic interactions is reached. As the affinity of porphyrin derivatives with water is reduced (upon decreasing their ionic nature), their interaction with surfaces competes with the solvated state. Clearly, this is possible only if the repulsion between porphyrins and the surface is not too high, rendering the energy term unfavorable. This simple model explains the different adsorption behaviors of the tetra- and dianionic porphyrins on glass-supported surfaces. Porphyrin Tetra Derivatives Adsorption on Glass: Effect of Polyelectrolytes. Our data strongly indicate that, to promote spontaneous adsorption of tetra-anionic H2TPPS on (1) and (2), the surface charge of the glass (1) needs to be completely reversed. Since we were not able to achieve this result by depositing ZnO layers, we decided to test positively charged polyelectrolytes.34−37 In particular, poly-L-lysine (PLL) is herein used to promote spontaneous H2TPPS adsorption on (1). PLL is an extensively used polyelectrolyte in which every amino acid unit has an amino side chain that is quite strongly basic (pKa ∼ 10.5). Hence, PLL is positively charged even in moderately alkaline media and accordingly is used as a scaffold to promote, through electrostatic interactions between the negatively charged peripheral groups of H2TPPS, stacking interactions between porphyrins themselves.38 Therefore, substrate (1) has been treated with PLL to demonstrate the possibility to trigger the glass surface charge by controlling and rationalizing PLL deposition. PLL-treated glass (3) samples was prepared by dipping (1) in PLL solutions (1 mM) having different pHs, ranging from pH = 5 to pH = 10 (Scheme 2, step 1). After 1 day in PLL, samples (3) were washed, dried, and dipped in H2TPPS solution at pH 7 (Scheme 2, step 2). After 2 h, H2TPPS-treated

samples were washed, dried, and optically analyzed by UV−vis spectroscopy (Scheme 2, step 3). Because of the high affinity between H2TPPS and PLL, the amount of porphyrin immobilized on (3) will depend on the amount of PLL deposited on glass. H2TPPS will, thus, act as a probe to estimate the affinity between (1) and PLL upon varying pH. At pH > 2, (1) is anionic and PLL is extensively protonated; then, according to mere electrostatic considerations, a high affinity between (1) and PLL should be expected upon decreasing pH. The UV−vis adsorption spectra of samples (3) after dipping in H2TPPS confirm the effect of PLL to promote H2TPPS adsorption (Figure 5). It is evident how PLL treatment, at different pH values, strongly affects the final outcome of the porphyrin adsorption. In particular, at pH = 5, a faint adsorption (Figure 5, black line) is detected on (3), while an increasing amount of porphyrin is observed upon increasing the pH of the PLL solution from pH = 7 (λMAX = 420.6 nm; Figure

Figure 5. UV−vis spectra (Soret region) of sample (3), treated with PLL at different pHs (step 1), after H2TPPS adsorption (step 2). A negligible signal is observed working with PLL at pH = 5, whereas a broad band peaked at λMAX = 420.6 nm is observed at pH = 7. At pH = 10, two components (at λMAX = 409 and 420.6 nm) are clearly distinguishable in the Soret region. 17662



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case, it is plausible to hypothesize that both solvophobic and electrostatic contributions might play a role in the (polar) porphyrin adsorption process onto a solid surface. The investigated water-soluble porphyrin tetra derivatives (H2T4 and H2TPPS) can remain as monomers in solution or move to the solid phase as assembled layers onto oxide surfaces. It is not the aim of the present paper to speculate about the energetic cost experienced by porphyrin moving at the liquid−solid interface. However, no doubt, electrostatic forces clearly figure prominently in this mechanism. In this balance between dispersive forces, surface charge modulation of glass is fundamental, and the presented results not only reinforce this hypothesis but validate two suitable approaches to control the robustness of the noncovalent surface functionalization process. The first one is based on the hypothesis that, by reducing the number of charged substituents (H2TPPS vs DPPS), the porphyrin solvophobic character will prevail, amplifying hydrophobic interactions between porphyrin and inorganic surfaces. The second approach regards the modulation of the glass surface charge by triggering the thickness and chemical nature of deposited overlayers (ZnO or PLL). As to the glass substrates (1), anionic porphyrins strongly repel their negative surface charge and ZnO deposition is able only to slightly modify the charge distribution. Evidence of the reduced negative overall charge of (2) is demonstrated by its increased affinity for DPPS and by the decreased affinity for H2T4 (as for comparison of the amount of porphyrin desorbed from (2) with respect to the amount desorbed from (1)). The observed behavior underpins the heretofore assumption that the nanosized thickness of ZnO layers (∼200−400 nm) is not effective in reversing the negative charge of the thicker (1 mm) glass substrate. This factor has a strong impact on the strength of the oxide surface−porphyrin interaction and allows for foreseeing the possibility of fine-tuning the electrostatics of the overall solid support (glass + ZnO) by dimensionally sizing the bulk support versus the deposited layers’ potentials. Reasonably, by increasing the ZnO thickness, it will be possible to immobilize increasing fractions of H2TPPS. However, this strategy is strongly affected by process limitations, since MOCVD is not effective in growing ZnO layers whose thickness is comparable to the substrate one. The use of cationic polyelectrolytes (PLL), thus, is more effective in modulating the surface (negative) charge of (1). PLL has been selected as the matter of choice since it has been used as an organizing receptor to promote aggregation of H2TPPS in solution.38 We, thus, foresee the possibility to transfer at the solid state the porphyrin chemistry, widely investigated in solution, to exploit its full potential in chemical sensing and devising. Surprisingly, PLL can be extensively immobilized on (1) at high pH (pH = 10) rather than at low pH (pH = 5). These results cannot be explained in terms of density of positively charged sites on the polylysine chain, which is definitely higher at pH = 5 than at pH = 10. Therefore, glass surface charge modulation upon varying pH cannot be neglected (Scheme 3). Accordingly, the glass surface is barely charged near its isoelectric point (pH 2.0), while its (negative) charge density increases upon increasing the pH.40 As to PLL, its hydrophilicity changes according to the polypeptide adopted conformation. At low pH, PLL exists as a random coil, whereas, by increasing the pH (pH > 10), the random-coil to α-helix transition is observed.41 The random-

5, green line) to pH = 10 (λMAX = 409 nm and 420.6 nm; Figure 5, red line). At pH = 10, a broad Soret band with two components centered at λMAX = 409 nm and λMAX = 420.6 nm is clearly detected. This spectral behavior indicates porphyrin aggregation at the surface.30,37 In fact, the component at λMAX = 409 nm can be attributed to H aggregates formation, whereas the one at λMAX = 420.6 nm is typically observed upon adsorption of porphyrin molecules onto inorganic surfaces.30,31,36,37 The pH of the H2TPPS solution is another important parameter that strongly drives the entity of porphyrin immobilization onto PLL-treated surfaces, accordingly to what was observed with PLL in solution.38 In fact, as not all the lysine residues interact with the solid surface, the remaining fraction is available for interaction or reaction with oppositely charged molecules from solution, according to the related protonation. Therefore, by dipping (3) in H2TPPS solution at pH = 7, a strong porphyrin adsorption is observed, due to an extensive lysine residues protonation, whereas at pH = 10, a faint absorption is detected, due to lysine residues deprotonation (Figure 6).

Figure 6. UV−vis spectra (in the Soret region) of (3) after dipping in H2TPPS at pH = 7 (red line) and pH = 10 (green line). UV−vis spectrum of (1) after dipping in H2TPPS at pH = 7 (black line) is shown for comparison.

PLL prototonation/deprotonation can be used to selectively release H2TPPS from the glass surface without affecting the polyelectrolyte layer stability. The reversibility and stability of the glass/PLL system has been proven effective by several dipping (in H2TPPS pH = 7) and washing (in water at pH = 13) cycles.

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DISCUSSION The presented results shed light on the complexity of tuning noncovalent immobilization of porphyrin onto oxide surfaces in water, since water represents the medium of choice for dispersive interactions. It should be reminded that, despite the presence of polar side groups (which convert the porphyrin chromophore into a water-soluble molecule), porphyrins display a global hydrophobic character. An increase in the number of sulfonated phenyl rings increases the water solubility (due to the prevalence of solute-water interactions), thus decreasing their aggregation tendency (solvophobic effect).39 In the studied 17663


The Journal of Physical Chemistry C Scheme 3. Schematic Drawing of the Charge Modulation of (1) and PLL by Changing pHa

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

Article

S Supporting Information *

SEM analysis of ZnO film deposited on glass by MOCVD; UV−vis analyses of H2T4, H2TPPS, and DPPS solutions; desorption experiments; and XPS surface analysis of ZnO deposited by MOCVD on glass substrates. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

a

Upon increasing the pH, the glass surface charge becomes more negative, while the PLL protonation (and the related positive charge) is reduced.

*E-mail: [email protected]. Fax: ++39 095580138. Phone: + +39 0957385126. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. All authors contributed equally.

coil conformation (with ionized side chains) is highly hydrated and thus is more stable in water and less prone to interact with hydrophobic surfaces than the α-helix conformation.42 Therefore, we cannot exclude the role of PLL conformation, also in terms of charge exposure, in the immobilization mechanism onto the glass surface. Work is ongoing to further investigate this aspect.

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Notes

The authors declare no competing financial interest

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ACKNOWLEDGMENTS The authors thank Ministero dell’Istruzione, dell’Università e della Ricerca (MIUR), for financial support through the FIRB “ITALNANONET” (RBPR05JH2P) project.

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CONCLUSIONS

To conclude, we demonstrated a straightforward, controllable, and environmentally friendly noncovalent functionalization of oxide surfaces. The obtained results provide an interesting insight into the full potential of electrostatic interactions to successfully functionalize inorganic surfaces using water-soluble porphyrins. In particular, the successful use of water-soluble species will hinge on the competition between retention at the solid state rather than in solution. Experimental data strengthen the hypothesis that both solvophobic and electrostatic contributions might play a role in the (polar) porphyrin adsorption process onto a solid surface and provide evidence that support a model in which the electrostatics of the (anionic) glass substrate plays a central role despite the (cationic) potential of a nanosized ZnO layer, reasonably due to the different thickness scaling (macroscale vs nanoscale). Polyelectrolytes are effective in promoting the glass charge reversal, thus allowing for an efficient and stable immobilization of a tetra-anionic porphyrin on a surface, but many process parameters strongly affect the overall porphyrin adsorption. In particular, we demonstrate, for the first time to our knowledge, the importance of pH to optimize polylysine (PLL) deposition on glass. At high pH, despite the reduced lysine residues protonation, PLL provides both an extensive coverage as well as a more suitable charge exposure for H2TPPS adsorption than that observed at pH = 7. Therefore, the success of the noncovalent surface functionalization in water requires a complex tuning of many factors, among them porphyrin solubility and solvophobicity, surface chemical and morphological complexity, overall charge distribution in solid and liquid phase, and molecular conformation control. The presented noncovalent functionalization is proven effective in terms of process reproducibility and control as well as robustness of the assembled layers, thus resulting in a promising route for future development in nanotechnology and material science.

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