Qingyun Cai

Humidity effect on the monolayer-protected gold nanoparticles coated chemiresistor sensor for VOCs analysis

Two gold-thiolate monolayer-protected nanoparticles were synthesized and used as interfacial layers on chemiresistor sensors for the analysis of violate organic compounds (VOCs). Toluene, ethanol, acetone and ethyl acetate were chosen as the target vapors. Both the resistance and capacitance were measured as the function of analyte concentrations. The effect of humidity on the sensor sensitivity to VOCs was investigated. The sensitivity decreases with humidity increasing, depending on the hydrophobicity of the target compounds. Less effect was observed on the higher hydrophobic compounds. While the relative humidity (RH) increased from 0 to 60%, the sensitivity to acetone decreased by 39 and 37%, respectively on the Au-octanethiol (C(8)Au) and Au-2-phenylethanethiol (BC(2)Au) coated sensors, while the sensitivity to toluene decreased by 12 and 14%, respectively. These results show that the sensors coated with hydrophobic compounds protected-metal nanoparticles can be employed in high humidity for hydrophobic compounds analysis. The resistance responses to VOCs are rapid, reversible, and linear, while the capacitance response is not sensitive and consequently not applicable for VOCs analysis. The response mechanism was also discussed based on the sensor response to water vapor. The capacitance response is not sensitive to the film swelling in dry environment.

Enzymatic assay of acid phosphatase and microanalysis of Cu2+ and Ag+ with a SAW-impedance sensor

Abstract Using adenosine-5′-monophosphate (5′-AMP) as a substrate, the surface acoustic wave (SAW)-impedance sensor has been successfully applied to detection of acid phosphatase and microanalysis of Cu 2+ and Ag + . The assay of acid phosphatase is based on the change in conductance of the solution caused by enzymatic reaction between 5′-AMP and acid phosphatase. The microanalysis of Cu 2+ and Ag + is based on its inhibitory action on enzyme. The Michaelis constant and the corresponding maximum initial rate are estimated to be 4.49 × 10 −3 mol l −1 and 8,841 Hz min −1 , respectively, at pH 4.5 and 28°C. The recovery of the sensor system ranges from 97.9–107.0% (n = 6). The experimental detection limit of enzyme is 0.7 mU ml −1 . The inhibition of Cu 2+ and Ag + is a noncompetitive one. The possible inhibition mechanism is discussed. The kinetic parameters and inhibition parameters are estimated. The detection limits of Cu 2+ and Ag + is 7.6 × 10 −7 m and 2.0 × 10 −8 M . Some other factors such as temperature, pH value, and active energy are also discussed.

Surface acoustic wave enzyme sensor applied to the kinetic assay of acid phosphatase

The acid phosphatase hydrolysis of phenylphosphoric acid (at pH 5 and at 25 °C) with a new type of liquid-immersion surface acoustic wave enzyme sensor (SAW) is described. A mathematical model was derived to estimate the kinetic parameters and to determine the concentrations of enzyme and substrate. The inhibition of the product to the enzyme activity was involved in the model. The Michaelis constant was estimated to be 1.17 × 10–4 mol l–1. The recovery of the sensor system was 97.4–104.4%(n= 6). The effects of temperature and pH on the enzyme activity were discussed. The proposed model compared favourably with the linear method which indicated that the model gave more precise results.

Determination of Organophosphorus Pesticides Based on the Inhibition of Lipase with a Surface Acoustic Wave Sensor System

Abstract A convenient and sensitive method based on the inhibition of the lipase-catalyzed hydrolysis by binding hydroxyl groups of lipase with organophosphorus pesticide is described for the enzymatic determination of organophosphorus pesticides. Employing the method proposed, the frequency response is proportional to organophosphorus pesticide concentration over the range of 250 ng/ml∼1.25 μg/ml. the detection limit is 58 ng/ml. The effects of temperature, pH value, incubation time and solvent are also investigated.

Rapid determination of ribonuclease and microanalysis of heparin with a SAW/conductance sensor

A new method using a surface acoustic wave (SAW)/conductance sensor has been described in this paper for rapid determination of ribonuclease (RNase) and microanalysis of heparin. The assay of RNase is based on the change in conductance of the solution caused by enzymatic reaction between ribonucleic acid (RNA) and RNase and the analysis of heparin is based on its inhibitory action on RNase. A linear relationship between frequency response and enzyme concentration is obtained and the detection limit of RNase is evaluated to be 0.17 mug ml(-1). The recovery of the sensor system ranges from 95.8 to 105.0%. The inhibition of heparin is a competitive one and the possible inhibition mechanism is discussed. The kinetic parameters and inhibition parameters are estimated. The calibration graph is rectilinear for </= 8 mug ml(-1) of heparin and the detection limit is 0.1 mug ml(-1). The influence of pH value on the inhibition of RNase by heparin has been investigated and the effects of Ca(2+), Mg(2+) are also discussed.

Rapid Determination of Acid Phosphatase by a Surface Acoustic Wave Impedance Sensor

Abstract A new type of surface acoustic wave impedance sensor based on a conductance-surface acoustic wave frequency reponse was applied to assay the activity of acid phosphatase. Using cytidine-5′-monophosphate (5′-CMP) as a substrate, the Michaelis constant and the corresponding maximum initial rate were estimated to be 4.13×10−3 and 9827 Hz/min, respectively. A linear relationship between frequency response and enzyme concentration was obtained in the range up to 250 μg/ml of acid phosphatase, at 25°C and pH 4.5. The experimental detection limit of acid phosphatase (within 1 min) was 0.9 min/ml and the recovery of the sensor system ranged from 94% to 106% (n = 6). Some other factors such as temperature, pH value and the effects of some metal ions were also discussed.

Enzymic analysis of L-lactate dehydrogenase (LDH) with a surface acoustic wave impedance sensor and its application in the determination of LDH activity in human serum

A new method is described for the determination of L-lactate dehydrogenase (LDH) using a surface acoustic wave impedance sensor. The assay of this enzyme is based on the change in conductance of the solution caused by enzymic reaction between lactate, NAD+ and LDH. A linear relationship between frequency response and enzyme concentration was obtained and the detection limit was 4.8 U ml–1. The Michaelis constant for lactate and the corresponding maximum initial rate were 0.03 mol l–1 and 13 kHz min–1, respectively. Influences such as temperature, pH value and interferents were also investigated. The method was successfully applied in the clinical assay of LDH activity in human serum. The results compared well with the reported values and support the clinical diagnosis.

SAW enzyme sensor applied to the determination of enzyme kinetic constants with the aid of a non-linear regression algorithm

A general program SAWBEH is proposed for the kinetic data evaluation of enzyme-catalysed hydrolysis by using the frequency shift response of an SAW sensor device. The Marquardt-Fletcher algorithm was employed as the minimum basis. The program was validated theoretically and compared with normal linear analysis methods by using synthetic data added with noise at different levels. The standard deviations of results obtained by the latter are about 100 times those obtained by the former. The system trypsin/benzoyl-L-arginine ethyl ester (BAEE) as a test model was studied at pH 7.6–7.8 and 32 °C. The Michaelis constant was estimated to be 6.34 × 10−5 M.