In situ, in-liquid, all-electrical detection of Salmonella typhimurium using lead titanate zirconate/gold-coated glass cantilevers at any dipping depth

Abstract

Most biosensing techniques are indirect, slow, and require labeling. Even though silicon-based microcantilever sensors are sensitive and label-free, they are not suitable for in-liquid detection. More recently lead zirconate titanate (PZT) thin-film-based microcantilevers are shown to be sensitive and in situ. However, they require microfabrication and must be electrically insulated. In this study, we show that highly sensitive, in situ, Salmonella typhimurium detection can be achieved at 90% relative humidity using a lead zirconate titanate (PZT)/gold-coated glass cantilever 0.7 mm long with a non-piezoelectric 2.7 mm long gold-coated glass tip by partially dipping the gold-coated glass tip in the suspension at any depth without electrically insulating the PZT. In particular, we showed that at 90% relative humidity and with a dipping depth larger than 0.8mm the PZT/gold-coated glass cantilever showed virtually no background resonance frequency up-shift due to water evaporation and exhibited a mass detection sensitivity of Deltam/Deltaf=-5 x 10(-11)g/Hz. The concentration sensitivities of this PZT/gold-coated glass cantilever were 1 x 10(3) and 500 cells/ml in 2 ml of liquid with a 1 and 1.5mm dipping depth, respectively, both more than two orders of magnitude lower than the infectious dose and more than one order of magnitude lower than the detection limit of a commercial Raptor sensor.

Fig. 1.

(a) An optical micrograph of the top-side view of the PZT/gold-coated glass cantilever, (b) phase angle vs. frequency resonance spectra in air (dashed-dotted line) and in PBS with a 1 mm (dashed line) and 1.5 mm (solid line) dipping depth, and (c) a schematic side view of the PZT/gold-coated glass cantilever, and the theoretical first- and second-mode flexural vibration waveforms of the PZT/gold-coated glass cantilever.

Fig. 2.

Rate of resonance frequency shift, Δf/Δt vs. dipping depth, x, at 90% relative humidity. The insert is a schematic of the PZT/gold-coated glass cantilever partially immersed in PBS with a dipping depth, x. Note that for x larger than 0.8 mm, Δf/Δt was essentially negligible and the background resonance frequency was stable with a fluctuation no more than 10 Hz.

Fig. 3.

Resonance frequency shift vs. time during (a) EDC/NHS modification and (b) antibody immobilization. The open circles (full squares) denote the results of cantilever at a 1 mm (1.5 mm) dipping depth. The open triangles denote results of the 5 MHz QCM. Note the large fluctuation in the resonance frequency during the detection was a signature of the binding events that took place during the detection, which was different from the stable background resonance frequency shown in Fig. 2 with a fluctuation no larger than 10 Hz.

Fig. 4.

Resonance frequency shift vs. time of Salmonella typhimurium detection with (a) 1 mm, (b) and (c) 1.5 mm dipping depth. The full squares, open circles, full diamonds, open up triangles, full up triangles, and open down triangles denote 1×10⁷, 1×10⁵, 1×10³, 500, 250 cells/ml, and the background, respectively. Note as similar to Fig. 3(a) and (b) the large fluctuations in the resonance frequency shift at higher concentrations were signatures of the binding events that took place during the detection, which was different from the decreased fluctuations in the resonance frequency shift at low concentrations and the stable background resonance frequency shown in Fig. 2 with a fluctuation no larger than 10 Hz.

Fig. 5.

SEM micrographs the PZT/gold-coated cantilever surface after 30 min detection in (a) 1×10⁷ cells/ml, (b) 1×10⁵ cells/ml, and (c) 1×10³ cells/ml S. typhimurium suspension.

Fig. 6.

−Δf vs. concentration at 90% relative humidity of the PZT/gold-coated glass cantilever where open triangles and full squares denote a 1 and 1.5 mm dipping depth, respectively, where Δf denotes the resonance frequency shift after 30 min. The infectious dose and the detection limit of a commercial Raptor are denoted by the open circle and open square, respectively.