Fluctuations of the number of adsorbed micro/nanoparticles in sensors for measurement of particle concentration in air and liquid environments

Abstract

A theoretical model of fluctuations of the number of adsorbed micro/nanoparticles in environmental sensors operating in air and liquids is presented, taking into account the effects of the mass transfer processes of the target particles in a sensor reaction chamber. The expressions for the total power of the corresponding adsorption-desorption noise, and for the corresponding signal-to-noise ratio are also derived. The presented analysis shows that the transfer processes can have a significant influence on the sensors limiting performance. The influence on both the fluctuations spectrum and the signal-to-noise ratio is estimated at different values of target particles concentration, functionalization sites surface density, and adsorption and desorption rate constants (the values are chosen from the ranges corresponding to real conditions). The analysis provides the guidelines for optimization of sensor design and operating conditions for the given target substance and sensor functionalization, in order to decrease the influence of the mass transfer, thus improving the ultimate performance (e.g., minimal detectable signal, signal-to-noise ratio) of sensors for particle detection. The calculations we performed show that it is possible to increase the signal-to-noise ratio for as much as two orders of magnitude by using optimization that eliminates the mass transfer influence.

Prikazan je teorijski model fluktuacija broja adsorbovanih mikro/nanočestica kod senzora za merenje parametara životne sredine, koji rade u vazduhu ili u tečnoj sredini. Model uzima u obzir uticaj procesa prenosa mase ciljnih čestica u reakcionoj komori senzora. Izvedeni su izrazi za snagu odgovarajućeg adsorpciono-desorpcionog šuma, kao i za odgovarajući odnos signal/šum. Prikazana analiza pokazuje da procesi prenosa mase mogu značajno da utiču na granične performanse senzora. Procenjen je uticaj prenosa mase na spektar fluktuacija i na odnos signal/šum pri različitim vrednostima koncentracije ciljnih čestica, površinske gustine funkcionalizujućih mesta i konstanti adsorpcije i desorpcije (izabrane vrednosti parametara su iz opsega koji odgovaraju realnim uslovima). Analiza omogućuje da se donesu zaključci potrebni za optimizaciju dizajna senzora i radnih uslova u smislu smanjenja uticaja prenosa mase, i da se time postigne poboljšanje graničnih performansi (npr. smanjenje minimalnog detektabilnog signala) senzora za detekciju čestica.