Differential measurements of light power variations through Si photodiodes in a bridge configuration for high-sensitivity chemical/biological optical sensing

This paper reports on a new optoelectronic technique based on the differential measurement of currents for the detection of small variations of the molar concentrations of chemical and biological substances by measuring changes of light power through two Si-photodiodes used in a bridge configuration. The solution exhibits high sensitivity, linear response and allows for the compensation of the initial bridge unbalance without changing its elements so optimising the output signal amplification and detection resolution. The proposed optoelectronic approach allows obtaining unique performances with respect to those ones achievable by using the standard synchronous demodulation technique for amplitude and phase measurements implemented by lock-in amplifiers. Moreover, the overall optoelectronic apparatus is simple and suitable for portable integrated sensor systems. Its main performances have been experimentally evaluated through a prototype PCB employing off-the-shelf components demonstrating the capability to detect light power variations with a settable maximum sensitivity of 300 mV/nW with a resolution of about 3 pW. Furthermore, a comparison with amplitude measurements based on the standard synchronous demodulation technique performed by lock-in amplifiers has demonstrated that the proposed optoelectronic system allows for a sensitivity enhancement of 2.9 × 104. As a case example, the optoelectronic system has been employed to determine variations of the molar concentration of a Rhodamine B solution diluted in ethanol with a resolution of about 5 pM by performing optical absorption measurements.