Balanced Laser Transmission Spectroscopy Based on a Tunable Gain Double Channel LIA for Nanoparticles Detection in Biomedical Applications

Abstract

This paper reports on the design, fabrication and characterization of a double channel, tunable gain Lock-in Amplifier (LIA) operating with voltage input pulses provided by two Si photodiodes that measure the power variations of 10ns laser pulses of a 10Hz repetition rate Q-Switched Nd:YAG tunable laser equipped with an optical parametric oscillator and second and third harmonic generation crystals. This laser is used to perform laser transmission spectroscopy measurement to evaluate both the concentration and dimension of nanoparticles for biomedical and biophysics applications. Nowadays, the challenge is to investigate the role of nanoparticles in activating biological processes when their concentrations are less than 109 particles/ml and/or their size ranges from few tens to few hundreds of nanometers. The laser transmission spectroscopy is a powerful technique to investigate these topics of research and is based on the measurement of the transmittance through the sample containing the nanoparticles (i.e., the signal channel) against that one through a sample with no nanoparticles (i.e., the reference channel). When the nanoparticles size and/or concentration are small, also the light scattering process that influences the transmittance is small: the value of the signal channel approaches that one of the reference channel. Thus, in this case, it is of paramount importance to develop methods to perform measurements with very low indetermination. The proposed double channel, tunable gain LIA is a solution to this problem since it allows to implement a new balanced laser transmission spectroscopy method. Before performing the laser transmission spectroscopy of the samples of interest, a calibration curve is accomplished for each wavelength of the laser beam passing through the signal and reference channels both in absence of nanoparticles. Under these conditions, the LIA gain is varied to achieve a ratio between the light power passing through the two channels close to 1. This avoids any experimental artifact due to the optical components that drive the laser beam along the two channels. Once verified that the calibration curve remains unaltered in time, the balanced laser transmission spectroscopy method was used to determine the wavelength dependent extinction coefficient of NIST standard polystyrene particles suspension. Respect to the method of the double ratio conventionally used for these measurements, the proposed balanced technique decreases the extinction coefficient relative error up to a factor 8. Moreover, the calculated particle size and concentration was found equal to 510±10nm and (1.18±0.08)×109 particles/ml, respectively. The indetermination of the particle size value respect to the nominal one is equal to 0.78% that is 4 times lower than the corresponding value calculated by the laser transmission spectroscopy using the double ratio method.