Impedance analysis of an electrode-separated piezoelectric sensor as a surface-monitoring technique for gelatin adsorption on quartz surface

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Abstract

The early events pertaining to gelatin adsorption and desorption onto quartz surfaces were studied, employing an electrode-separated piezoelectric sensor (ESPS). The adsorption of gelatin on a quartz crystal surface corresponds to a mass increase, which can be monitored in real time by the changes in the impedance parameters of the ESPS. It was shown that the adsorption of gelatin on a quartz surface is partly irreversible with respect to the dilution of the bulk phase. The observed adsorption kinetics is compatible with a mechanism that involves adsorption, desorption, and transformation from a reversible adsorption state to irreversible one. A progressive approach method was established to simulate the adsorption process. The adsorption densities and kinetic parameters in the early adsorption process were obtained from the responses of the ESPS in the adsorption process. The influence of pH and ionic strength was tested. A comparison with the Langmuir adsorption model was made.

Introduction

Biomacromolecules such as proteins show a strong tendency to physically adsorb at liquid/solid interfaces due to favorable van der Waals, ionic, and polar interactions [1]. Adsorption of proteins at solid/liquid interfaces is an important and challenging interdisciplinary field of the natural sciences [2], [3], [4]. Understanding the mechanism and the processes involved in protein interaction at the solid/liquid interface is essential for a number of medical and biochemical applications, including implantation and function of biomaterials/medical implants in soft and hard tissue and in the bloodstream, fouling of contact lenses, food processing equipment; extracorporeal therapy, drug delivery, bacteria and cells in culture, and a variety of medical sensors and other diagnostic tools. Effective adsorption of proteins is important in protein separation, immobilization of enzymes, biosensors and protein chips, etc. But in other processes, e.g., membrane filtration, biofouling of membranes, and artificial organs, protein adsorption is highly undesirable. A number of techniques have been utilized in the study of protein adsorption, including ellipsometry [5], [6], [7], atomic force microscopy [8], [9], rediabeling [10], circular dichroism [11], ATR-FTIR spectroscopy [12], [13], vibrational sum frequency spectroscopy [14], surface plasmon resonance (SPR) [15], [16], [17], [18], total internal reflected fluorescence (TIRF) [19], [20], [21], and quartz crystal microbalance (QCM) [22], [23], [24], [25], [26], [27], [28]. So far, the techniques of ellipsometry, ATR-FTIR, SPR, TIRF, and QCM have demonstrated real-time kinetic measurements of protein adsorption.

The QCM is well known for its very high sensitivity to slight mass loading variation on the electrode surface of the quartz crystal. It is suitable for time-resolved in situ measurements and can be manufactured in large quantities at low costs. QCM sensors allow on-line and direct detection of label-free proteins, thus saving time and providing the opportunity to study the kinetics of the adsorption process. Usually, a QCM sensor is constructed with an AT-cut quartz disc sandwiched between two metal electrodes, which induce the quartz crystal to oscillate at a frequency in the MHz region. In this work, a modified QCM device, an electrode-separated piezoelectric sensor (ESPS), was used.

In the design of the ESPS, a naked quartz surface is in directly contact with the liquid phase [29], [30], [31], [32]. This configuration offers the advantages of long life of the sensor and convenience of monitoring the mass change on the quartz surface. The quartz surface is an important interface in capillary electrophoresis, biochips, and measurement cells in UV spectrometry or fluorometry. On the other hand, the quartz surface has good similarity to silicon dioxide or silica surfaces extensively used as model surfaces in adsorption investigations.

Gelatin is a polydisperse polypeptide derived from the structural protein collagen. It is one of the most commonly used polymers in industry and has become the preeminent polymer of industrial importance because it is an excellent colloidal stabilizer and produces optically transparent, thermoreversible gels at near body temperatures. Gelatin is ubiquitous in the photographic [33], food [34], and pharmaceutical industries [35] and as an adhesion agent for depositing different types of small particles from aqueous dispersions onto substrates [36]. Gelatin is obtained by acid or alkali denaturation of collagen, which consists of three polymer chains in a triple helix and contains both acidic carboxyl and basic amino groups which, in solution, transform into the corresponding negatively and positively charged groups [37]. The adsorption behavior of gelatin is expected to complex. Some recent experimental investigations of gelatin adsorption onto mica [38], phosphatidyl choline-coated silica [39], fumed silicas [40], [41], carbon particles [42], and polystyrene latices [43] have been reported. Several experimental methods have been employed for studying gelatin adsorption, including solution depletion [44], [45], ellipsometry [46], surface force apparatus [47], [48], and small-angle neutron scattering [49]. Despite much work devoted to gelatin adsorption, much still remains to be done to understand the adsorption mechanisms.

In this work, the early events pertaining to gelatin adsorption and desorption onto quartz surfaces were studied, employing the ESPS device. To characterize the ESPS more completely, an impedance analysis method has been employed. In the impedance analysis method, the magnitude and phase of the impedance of the piezoelectric sensor were scanned with frequency from the externally applied voltages. Electrical properties of the quartz crystal can be found from the impedance–frequency curves [50], [51], [52], [53], [54]. An advantage of the impedance analysis method is its ability to give some insight into the viscoelastic behavior of the adsorbed materials, which is achieved by monitoring the motional resistance. During the adsorption of gelatin on quartz crystal surfaces, the resonant frequency of the ESPS decreases with increasing adsorption mass. The frequency change arising from the nonmass effect was distinguished from the correlations of motional resistance and resonant frequency. The influence of surface roughness was calibrated by dye adsorption. A modified adsorption kinetics model was developed to describe the experimentally observed early adsorption behavior of the gelatin on the quartz surface. By using a progressive approach method, adsorption kinetics parameters were estimated from the responses of the ESPS in the adsorption process. The influence of pH and ionic strength was tested. A comparison with the Langmuir adsorption model was made.

Section snippets

Apparatus and reagents

The configuration of the ESPS used was illustrated in Fig. 1. A 5 MHz AT-cut quartz crystal disc of 25.4 mm in diameter was used. A keyhole-shaped gold electrode with a diameter of 12 mm was evaporated on the center of one side of the quartz disc on a chromium adhesion layer. The quartz disc was flexibly fixed and sealed to a glass detection cell by silicon glue with the side of the bare quartz facing the liquid phase and the other side with the gold electrode in the air in a closed chamber. A

Adsorption of gelatin on quartz surface

The QCM is a mass sensor with high sensitivity to mass changes on the surface of an oscillating quartz crystal. When the Sauerbrey relation holds, the adsorbed mass (Γ) is directly proportional to the change in frequency (Δf) and is given by Γ=Δm/A=αΔf, where Δm is the mass adsorbed onto the active area of the quartz crystal and A is the geometric area of the active region of the quartz crystal disc. For a 5-MHz AT-cut quartz crystal, α=17.8ng cm−2Hz−1 was estimated according to the Sauerbrey

Conclusions

We concluded that the ESPS is a useful device to monitor the initial kinetics of adsorption of proteins on quartz surface. Combined with mathematical models, valuable information about the interaction processes can be obtained. It was shown that the early adsorption process of gelatin on quartz surfaces can be described by a model that involves reversible adsorption and transformation from the reversible state to the irreversible one. Using a progressive approach computing method, the rate

Acknowledgments

This work is supported by the National Science Foundation of China (No. 20275021) and open foundation of the State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University.

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