Rapid, sensitive and selective detection of bioparticles is of prime importance in point of care diagnostic devices and various micro- and nano-systems [1] relying on microfluidics are being developed. This paper reports a design of a new optoacoustic microfluidic device which can quantitatively detect the concentration of micron sized bioparticles in a solution. In our previous work [2], we have introduced a new sensing scheme utilizing surface acoustic wave (SAW) detection of the photoacoustic (PA) signal generated from optical absorption of bioanalytes present in the microfluidic channel. In the current work, we combine a SAW based particle concentration on a microfluidic reservoir with the SAW-PA sensor to enable rapid, quantitative and sensitive microparticle detection (polystyrene) on a single piezoelectric substrate. The detection of analytes from the bulk of the solution, along with the mechanical scanning, particle concentration and continuous fluid flow, highlights the device capability to handle clinically relevant sample volume (millilitres) in comparison to the low throughput (microlitre sample) of the existing devices.
A schematic view of the device is illustrated in Fig. 1. Orthogonal interdigital transducers (IDT) on the 1280 YX lithium niobate (LiNbO3) piezoelectric substrate is used for actuating and sensing. An IDT excited at 15.8 MHz (labelled ‘A’) oriented in the Y direction is used for particle concentration inside the microfluidic reservoir. The IDT at centre frequency of 10 MHz oriented in the X direction is used for sensing. The numerical simulation results confirms that the SAW-PA frequency for a 10 μm (diameter) polystyrene particle is sensitive for frequencies less than 50 MHz. Table I shows the SAW device specifications. A pulsed laser at repetition rate of 10 Hz at a wavelength of 532 nm is used, with an optical spot size of 200 μm. A mechanical scanner focusses the laser spot across the particles located inside the microfluidic reservoir. Fig. 2 shows the schematic of the experimental setup used for sensing. Black polystyrene microparticles with an optical absorbance of ~ 0.7 at 532 nm is used as the sensing sample for the experiment.
Fig. 3 shows 10 μm particles aggregating after SAW exposure for 40-50 sec. The particles are concentrated at a vertical height close to the liquid-air free surface. The preconcentration before detection bounds the mechanical scanning to only half of the cavity, thereby reducing the detection time. Fig. 4 shows the experimental results for the detection of the 10 μm particles for a concentration varying from ~10-200 particles per 10 μL. The power spectral density of the SAW-PA signals for varying particle concentration demonstrates a quadratic response, with a detection of 7 particles in 10 μL of solution. The sensitivity can be improved further to a single particle by optimizing the SAW design to match the acoustic frequency generated due to microfluidic channel resonance. Furthermore, utilizing dual SAW IDTs aggregates the particles at a preferred location, thereby eliminating the mechanical scanner. Thus, a rapid (few sec) and sensitive (single) particle detection device could be envisaged, comparable to the 3D droplet detection device[3].
