Development of a Photothermal Absorbance Detector for Use with Microfluidic Devices

Abstract

The development of a photothermal absorbance detector for use with microfluidic devices is described. Unlike thermooptical techniques that rely on measuring refractive index changes, the solution viscosity is probed by continuously monitoring solution conductivity. Platinum electrodes microfabricated on a quartz substrate and bonded to a substrate containing the microchannels enable contact conductivity measurements. The effects of excitation frequency and voltage, electrode spacing, laser power, and laser modulation (chopping) frequency were evaluated experimentally. In the current configuration a limit of detection of 5 nM for DABSYL-tagged glucosamine was obtained using long injections (to give flat-topped peaks). This corresponds to an absorbance of 4.4 × 10−7 AU. Separation and detection of DABSYL-tagged glycine, proline, and tryptophan is also shown to demonstrate the feasibility of the method. In addition, simulations were used to investigate the applicability of the technique to small volume platforms.

Development of a conductivity-based photothermal absorbance detection microchip using polyelectrolytic gel electrodes

“…showed that microfluidic-based photothermal absorbance detection could be achieved using contact conductivity measurement with platinum electrodes and a high frequency AC excitation signal. [32] A marked increase in the overall sensitivity and agreement with theoretical calculations was shown, however, a limitation to the applied excitation signal due to electrode degradation was observed. While a limit of detection (SNR = 3) of 5 nM for DABSYL-tagged glucosamine was achieved, any misalignment between the detection region and the laser beam could cause a loss in signal intensity and possibly damage the electrodes.…”

“…While the maximum signal intensity continued to increase with excitation voltage, a signal of 5 V p–p was chosen to allow comparison to previously published results using a metal electrode based microchip. [32] With the metal electrode microchip, the photothermal SNR increased with excitation frequency up to the lock-in amplifier’s limit (102 kHz), however this was not seen with the PGE microchip (Figure 2b). Although the photothermal signal intensity increased linearly with the excitation frequency, the baseline noise also increased possibly because the background signal nulling through the PGEs has a frequency limit.…”

“…[31,32] Briefly, 6 mM DABSYL chloride in acetone and 1 mM glucosamine in water were dissolved in 50 mM carbonate buffer at pH 9.0 in a 1:1 ratio. This solution was then heated in a water bath at 75 °C for 12 minutes.…”

“…The electronic components for the 3-electrode PGE microchip were similar to those used previously. [32] Two DS345 digital function generators (Stanford Research Systems) were used to supply the AC excitation signal to the conductivity cell. All measurements were taken under zero background conditions by adjusting the phase and voltage of the function generators to null the background conductivity, using a master-slave configuration.…”

Development of Tl+/Na+ ion-exchanged single-mode waveguides on silicate glass for visible-blue wavelengths applications

“…The property of light absorption by molecules is one of the most used and easy to implement. It can be used to create photothermal lenses whose refractive index varies with the analytes concentration [2,3]. As molecules present numerous absorption bands in the visible spectral range, there is a need for a shift of the working wavelengths from the usual near Infra-red [1.3À 13À 1.5] μm telecom band towards the [0.4À 04À 0.8] μm visible one.…”