Fast-response and low-loss surface acoustic wave humidity sensor based on bovine serum albumin-gold nanoclusters film

The impact of ambient air humidity on surface acoustic wave (SAW) propagation in structures consisting of bovine serum albumin (BSA) and bovine serum albumin - gold nanoclusters (BSA-Au NCs) films deposited on YZ lithium niobate (LiNbO3) substrate has been investigated. The BSA-Au NCs film deposited on the surface of LiNbO3 significantly reduced the low-humidity value of the SAW transmission loss and considerably improved its long-term stability as compared to the BSA film. The amplitude of the output signal of the BSA-Au NCs on LiNbO3 SAW delay line was measured as a function of relative humidity (RH) at the steady-state conditions and as a function of time upon the step-like RH variation. An observed decrease in the transmitted SAW amplitude with increasing RH was explained in terms of the acoustoelectric attenuation. The response and recovery times of the structure with BSA-Au NCs on LiNbO3 SAW delay line upon abrupt humidity changes were on the order of 0.2 s. No impact of RH variation on the SAW propagation was observed in the structures with BSA-Au NCs films deposited on metalized LiNbO3 substrate surface. The time-stable, fast-response and low-loss BSA-Au NCs on LiNbO3 SAW humidity sensor has been applied for monitoring the human respiration by detecting moisture contained in the exhaled air.