Adsorption studies of dodecylamine at the quartz interface using an electrode-separated piezoelectric sensor

The adsorption process of dodecylammonium chloride (DAC) from aqueous solutions onto a quartz crystal interface was investigated in situ using an electrode-separated piezoelectric sensor (ESPS). Increasing amounts of DAC adsorbed onto a quartz crystal surface resulted in linearly decreasing oscillating frequencies of the ESPS. The adsorption density can be monitored by the frequency decrease. The adsorption density obtained by calculation using the Sauerbrey equation in the ESPS method is greater than that in solution depletion method. A calibration coefficient is added into the Sauerbrey equation to correct the influence of surface roughness of the quartz crystal on the adsorption density. The influence of solution properties on the adsorption density measurement was discussed. A dependence of the adsorption density on pH was reported. Received: 17 April 1997 / Revised: 1 December 1997 / Accepted: 7 December 1997     

Adsorption studies of cationic starch onto quartz surface through an electrode-separated piezoelectric sensor

In the construction of an electrode-separated piezoelectric sensor (ESPS), the quartz surface is in direct contact with the liquid phase. The negatively charged quartz crystal surface can adsorb cationic starch. This adsorption process was in situ monitored from the frequency shift of the ESPS. It was shown that the adsorption of cationic starch onto the quartz surface is reversible with respect the dilution of the bulk phase. The adsorption behavior can be described by Langmuir model. The adsorption density and kinetics parameters were estimated from the frequency responses of the ESPS. The influence of pH and ionic strength on adsorption parameters was investigated. It was shown that the influence of pH on the adsorption rate was slight. With increasing ionic strength, the rate constants for adsorption and desorption increase, but the adsorption equilibrium constant and saturation adsorption density decrease. The adsorption equilibrium constant and adsorption density reach a maximum in buffer of pH 10.     

Silver-selective sensor using an electrode-separated piezoelectric quartz crystal modified with a chitosan derivative.

Silver(I) adsorbed selectively onto a quartz plate modified with N-(2-pyridylmethyl)chitosan in an ammonium chloride buffer solution containing EDTA, and the frequency of the quartz plate increased. It was supposed that the increasing frequency was caused by the desorption of adsorbed water on the chitosan derivative, which was induced from the reaction of silver(I) with the chitosan derivative. The concentration of the buffer, pH, temperature, conductivity and eluent affected the frequency shift resulting from the adsorption of silver(I). The frequency decreased at a conductivity lower than 2.2 mS/cm, and increased with increasing conductivity above this value. The frequency shifts caused by the adsorption of silver(I) were proportional to the concentration over the range 10-80 nM of silver(I), and the correlation coefficient was 0.9969. The detection limit and the relative standard deviation at 50 nM for five times were 6 nM and 3.4%, respectively. The proposed method was simple while showing higher sensitivity and selectivity.     

Gravimetric monitoring of adsorption behavior of tetrakis(methylpyrdiniumyl)porphyrin onto quartz crystal surface using an electrode-separated piezoelectric sensor

The adsorption behavior of 5,10,15,20-tetrakis (4-N-methylpyridiniumyl)-porphyrin (H₂TMPyP) from aqueous solutions onto a quartz crystal interface was investigated in situ using an electrode-separated piezoelectric sensor (ESPS). With H₂TMPyP adsorbed onto quartz crystal surface of the ESPS, its oscillating frequency decreases linearly with increasing adsorption amount. The adsorption densities obtained in the ESPS method were greater than those determined in a solution depletion method. The influence of surface roughness of quartz crystal and bulk solution properties on the measurement of adsorption density in the ESPS method was discussed.     

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

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.     

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

This paper describes the measurement of the kinetics of adsorption of sodium dodecyl sulfate (SDS), an anionic surfactant, onto a quartz surface with a pre-adsorbed layer of Ca2+ as an ion bridge, using an electrode-separated piezoelectric sensor (ESPS). An impedance analysis method was employed to characterize the responses of the ESPS. The impedance and frequency parameters of the ESPS were examined as functions of the conductivity, permittivity, viscosity and density of the liquid. The adsorption process of SDS onto the quartz surface resulted in an increase in both the mass and energy dissipation of the oscillating quartz crystal. The adsorption densities could be estimated by the ESPS method after taking into consideration the effects of surface viscosity and roughness. The adsorption and desorption rate constants of SDS onto the quartz surface were calculated as ka = (88.1 +/- 0.26) mol(-1) L s(-1) and kd = (4.92 +/- 0.53) x 10(-3) s(-1), respectively, based on the Langmuir model. ESPS was shown to be a powerful means of examining anionic surfactant adsorption to the solid/liquid interface.     

Impedance and frequency characteristics of electrode-separated piezoelectric quartz crystals in liquids

The electrical equivalent circuit parameters of an electrode-separated piezoelectric quartz crystal (PQC), where the gaps between the electrodes and the crystal plate were filled with various liquids, were evaluated from admittance measurements and were compared with those of a normal PQC with Au electrodes adhering to the crystal plate. The resistance parameter in the equivalent circuit of the electrode-separated PQC was more sensitive to the specific conductivity, density and viscosity of liquids than was that of the normal PQC. This parameter, unlike that of the normal PQC, was also affecteded by the permittivity of the liquid. The effect of the liquid properties on the other parameters in the equivalent circuit and the series resonant frequency are also discussed.     

Adsorption behavior of metal ions onto a bovine serum albumin (BSA) membrane monitored by means of an electrode-separated piezoelectric quartz crystal.

The interaction between metal ions and bovine serum albumin (BSA) was studied by using a piezoelectric quartz crystal (PQC) arranged in the electrode-separated configuration. A silanized surface of the PQC was coated with a BSA membrane via a coupling reaction with glutaraldehyde. The frequency shifts obtained from PQC coated with a BSA membrane suggested that various kinds of metal ions could be adsorbed onto the BSA membrane from aqueous solutions containing a low concentration of metal ions (2 or 10 micromol dm(-3)), only when the BSA was denatured with an alkaline solution. Anionic species of Pt(IV) and Au(III) were adsorbed onto the denatured BSA membrane from an acetic acid solution at pH 2.2, and cationic species of Cd(II), Zn(II), Co(II), Ni(II), Cu(II), and Ag(I), and cations, such as Ca2+, Ba2+, and Mg2+, were adsorbed from ammonia buffer at pH 9.5, whereas Al(III), Cr(III), Fe(III), Hg(II), and Pb(II) were hardly adsorbed. The adsorption mechanisms of these metal ions are discussed, based on the electrostatic interaction between the metal ions and the denatured BSA membrane, and complex formation between the metal ions and amino acid residues of the denatured BSA. Further, the PQC coated with a denatured BSA membrane was applied to the determination of Pt and Cd, using large frequency shifts for Pt(IV) and Cd(II).     

Effects of longitudinal wave on the resonance behavior of an electrode-separated piezoelectric sensor in liquids

An electrode-separated piezoelectric sensor (ESPS) was constructed and its operation in the liquid phase was characterized. When the surface of the separated-electrode reflects the weak longitudinal wave (LW) back to the surface of the quartz disc where it was generated, the interaction of the reflected LW with the main thickness-shear wave of the quartz disc is a potential error source. In this study, the influence of the LW effect on the response of the ESPS was investigated by an impedance analysis method. It was shown that all the impedance response parameters shift periodically as a function of the distance between the separated-electrode and the quartz crystal. The LW effect depends markedly on the surface conditions, including the shape, area, position, angle and roughness, of the separated-electrode. When a rough, non-parallel or ball electrode was used, the influence of the LW could be decreased greatly. The influence of the air/liquid interface on the reflection of the LW was efficiently screened in the ESPS.     

Influence of oscillator phase on oscillation frequency and sensitivity of an electrode-separated piezoelectric sensor

Theoretical consideration was proposed for the influences of the oscillator phase on the response nature of an electrode-separated piezoelectric sensor (ESPS) to liquid properties and verified experimentally. For the oscillators with different phases, the oscillation frequency of the ESPS may increase, maintain nearly constant or decrease with increasing conductivity in low conductivity solutions. The sensitivity of response to density and viscosity increases slightly with increasing oscillator phase. The response to permittivity depends hardly on the oscillator phase. In addition, the dependence of the oscillation frequency of the ESPS on the supply voltage of the oscillator was explained. The ESPS was used to determine atropine sulfate with a detection limit of 1.6×10~7 mol/L.     

Relative humidity measurement using a coated piezoelectric quartz crystal sensor

Miniature piezoelectric quartz crystals, which are manufactured cheaply by photolithographic and chemical etching techniques, have a high mass sensitivity. AT-cut 10 MHz quartz crystals have been coated with four materials and used to measure the relative humidity in various gases. The coated crystal is used as a resonator in an oscillator circuit, the frequency of which varies as a function of the change in mass. Characteristics that determine the usefulness of the coatings, such as sensitivity, response linearity, response time, selectivity, hysteresis and ageing, were evaluated. The detector has potential for use as a hygrometer, provided that the appropriate coating is chosen for a specific application.     

Detection of P. aeruginosa using nano-structured electrode-separated piezoelectric DNA biosensor

A DNA biosensor for detection of Pseudomonas aeruginosa was set up based on the modification of two membranes (nano-TiO₂ and nano-TiO₂-polyethylene glycol hybrid membrane) to the ESPS surface. These two membrane materials were synthesized by sol-gel method. The detection was accomplished by modifying ss-DNA on the sensitive membrane and then hybridizing with their complementary strands from the P. aeruginosa in liquid phase. UV spectrum was used to identify the purity and concentration of extracted DNA; IR spectrum and SEM were used to characterize the properties of the membrane. The detection was highly improved by adoption of nanotechnology and hybrid membrane. Less than 3 h was sufficient. The detection linear range was from 10⁻¹ to 10⁻³ g l⁻¹ and the limit of detection was 10⁻⁴ g l⁻¹.     

High sensitive piezoelectric quartz crystal liquid density sensor

<正>We report for the first time a cleavage phenomenon in the resonant peak of a piezoelectric quartz crystal(PQC) in liquid phase.In the presence of a strong longitudinal wave effect,an additional resonant peak appears in the conductance-frequency curve.With gradually increasing liquid density,the additional peak moves from low to high frequency region then disappears.The frequency of the additional resonant peak is sensitive to the change in liquid density.The frequency shift of the additional peak is linear with the liquid density in a given range.For a 5 MHz PQC with a reflection distance of 16 mm for longitudinal wave,the sensitivity to liquid density is 2.61×10~6 Hz g~(-1) cm~3.The overlap between the primary resonant peak and the additional resonant peak causes a decrease in the intensity of the former and an increase in the intensity of the latter.In a combined impedance analysis method,the changes in surface mass loading,density and viscosity of the liquid were monitored simultaneously by a PQC sensor.     

Development of a chemical vapor sensor using piezoelectric quartz crystals with coated unusual lipids

Piezoelectric quartz crystal sensors were developed using lipids with various properties for highly sensitive detection of chemical vapors. Lipids with varying lengths of alkyl chains were coated onto 10 MHz AT-cut quartz crystal resonators and the response of these modified crystals to chemical vapors were measured. It was shown that hydrophilic compounds, such as ethanol and methanol, could be recognized efficiently by lipids having shorter alkyl chains, whereas lipids with longer alkyl chains showed affinity to more hydrophobic vapors, such as toluene, hexane and cyclohexane. Frequency changes caused by adsorption of alcohols could be enhanced when cholesterol was co-immobilized in the lipid layer. To confirm the assumption that the sensor-response might be affected by the properties of lipids derived from acyl chains, we have examined the effects of two types of newly synthesized unusual lipids on sensor response. When lipids having one triple bond each at different positions on their alkyl chains were coated onto quartz crystals separately, lower responses were observed compared to responses obtained for a sensor with immobilized, saturated phosphatidylcholine. Lipids containing -branched acyl chains, however, showed good affinity for organic vapors, and sensor responses improved 4–5-fold. Moreover, these sensors were shown to have sensitivity of the same order as the humans' sense of smell (10⁻⁵–10⁻⁶ w/w in liquid paraffin) when measured using standard odorants (isovaleric acid, skatole, etc.) for an olfactometry established in Japan.     

Adsorption of metal ions from solution onto a piezoelectric quartz crystal

Many metal ions are spontaneously adsorbed onto a piezoelectric quartz crystal and change the oscillation frequency. The pH ranges in which the metal ions adsorbed were just below that of precipitate formation as the ‘hydroxides’; frequency changes caused by adsorption were not observed in these pH ranges where the ‘hydroxides’ formed. Cationic organic reagents, such as crystal violet and methylene blue, were also adsorbed on the piezoelectric quartz crystal but non-ionic and anionic organic compounds were not. These results showed that the crystal was negatively charged on the surface.     

Kinetic determination of organic vapor mixtures with single piezoelectric quartz crystal sensor using artificial neural networks

A single piezoelectric quartz crystal coated with one kind of crown ether was applied to the simultaneous determination of binary acid and amine vapor mixtures. From the adsorption and desorption curves of analytes, which were somewhat different in shape, frequency shifts from ten time windows were taken as inputs for artificial neural networks (ANN). Prediction results were satisfactory for ANN in both sample sets. The average relative errors, for formic acid and acrylic acid were 5%, for n-butylamine and aniline, they were 3% with ANN respectively. The effects of number of neurons in the hidden layer of ANN on the performance of the network are also discussed.     

Adsorption of interacting oligomers and polymers at an interface

We discuss in a qualitative way the physical background of a recently developed polymer adsorption theory, in which all the possible chain conformations for interacting chain molecules near an adsorbing interface are taken into account. Any conformation is described as a step-weighted random walk in a lattice. Each step is weighted according to a segmental weighting factor that contains the adsorption energy (for segments in contact with the surface), the entropy of mixing, and the attraction or repulsion between segments and solvent molecules. A suitable computing method is used to calculate the contribution of all chain conformations to the concentration profile, to the adsorbed amount, to the fraction of trains, loops and tails, to the layer thickness, etc. The theory is valid for any chain length and any concentration in the solution.Results for various chain lengths are given. Oligomers have a low affinity for the surface, whereas polymer adsorption isotherms are of the well known high affinity type. Three concentration regimes can be distinguished. In (extremely) dilute solutions the molecules on the surface adsorb as isolated chains (the Henry region).     

Adsorption of ionic surfactants at an expanding air-water interface

A quantitative model for the kinetics of adsorption of ionic surfactants to an expanding liquid surface is presented for surfactant concentrations below and above the critical micelle concentration (cmc). For surfactant concentrations below the cmc, the electrostatic double layer is accounted for explicitly in the adsorption isotherm. An overflowing cylinder (OFC) was used to create nonequilibrium liquid surfaces under steady-state conditions. Experimental measurements of the surface excess for solutions of cationic surfactants CH3(CH2)n-1N+(CH3)3 Br- (CnTAB, n = 12, 14, 16) and the anionic fluorocarbon surfactant sodium bis(1H,1H-nonafluoropentyl)-2-sulfosuccinate (di-CF4) in the OFC are in excellent agreement with the theoretical predictions for diffusion-controlled adsorption for all concentrations studied below the cmc. For surfactant concentrations above cmc, the diffusion ofmicelles and monomers are handled separately under the assumption of fast micellar breakdown. This simplified model gives excellent agreement for the system C14TAB + 0.1 M NaBr above the cmc. Agreement between theory and experiment for C16TAB + 0.1 M NaBr is less good. A plausible explanation for the discrepancy is that micellar breakdown is no longer fast on the time scale of the OFC (ca. 0.1 s).     

Adsorption of nanoparticles at the solid-liquid interface

The adsorption of differently charged nanoparticles at liquid-solid interfaces was investigated by in situ X-ray reflectivity measurements. The layer formation of positively charged maghemite (γ-Fe(2)O(3)) nanoparticles at the aqueous solution-SiO(2) interface was observed while negatively charged gold nanoparticles show no adsorption at this interface. Thus, the electrostatic interaction between the particles and the charged surface was determined as the driving force for the adsorption process. The data analysis shows that a logarithmic particle size distribution describes the density profile of the thin adsorbed maghemite layer. The size distribution in the nanoparticle solution determined by small angle X-ray scattering shows an average particle size which is similar to that found for the adsorbed film. The formed magehemite film exhibits a rather high stability.