The previous study mentioned that nanoscale particles exhibit positive DEP at the frequency Ceritinib clinical trial window of low frequency [27], and it has been shown that their cross-over frequency is with respect to the product of the Debye length and the particle size [26]. When an AC voltage of 15 Vp-p at a frequency of 100 kHz was supplied to the quadruple electrode, the negative DEP force caused 5 μm to be concentrated in the middle area of the weakest electric field region. At this frequency, the fluorescent nanocolloids were induced with a positive DEP force that manipulated the fluorescent nanocolloids into the microparticle aggregate.

After applying voltage for 3 min, we switched the observation from a bright field to a fluorescent field. The result clearly showed that the DEP-formed microparticle aggregate exhibits check details an evident fluorescence phenomenon, as shown in Figure  3a,b. This process can be utilized to validate and illustrate that the fluorescent nanocolloids were effectively trapped into the bead-bead gaps of the assembled microparticles due to the amplified positive DEP force and also were trapped on the local surface of the microparticles. Figure  3b shows the nanoDEP trapping result under the same condition but at a lower concentration of fluorescent nanocolloids. Figure 3 Nanocolloid trapping mechanism. (a1) Five micrometers was induced with a negative DEP force to be concentrated

in the middle area. (a2) The DEP-assembled microparticle aggregate traps the fluorescent nanocolloids effectively, thus exhibiting an evident fluorescence phenomenon. (b1, b2) NanoDEP trapping result at a lower concentration of fluorescent nanocolloids.

Optimal conditions and on-chip SERS identification of bacteria The bacteria (S. aureus) was found to exhibit strong positive DEP (pDEP) at frequencies above 3 MHz and strong negative CYTH4 DEP (nDEP) below 2 MHz, while blood cells exhibited strong nDEP at frequencies below 500 kHz and strong pDEP behavior above 800 kHz. AgNPs were spiked into the prepared bacteria solution to adjust to a constant bacteria concentration of 107 CFU/ml with different AgNP concentrations. At frequencies below 2 MHz, all bacteria exhibited nDEP in the conductive medium with a conductivity of 1 mS/cm and were trapped in the middle of the electrode gap. Metal-based nanocolloids have been shown to exhibit a high positive DEP force at both low and high frequencies due to their high conductivity and polarizability [28]. Therefore, a voltage of 15 Vp-p at a frequency of 1 MHz was applied to simultaneously concentrate the bacteria using negative DEP and to trap the AgNPs by the bacteria assembly that produced the amplified positive DEP force. To investigate the optimal AgNP concentration in the bacteria solution for the enhancement of the Raman signal, the different AgNP concentrations of 2.5 × 10-7, 5 × 10-7, and 1 × 10-6 mg/μl were adjusted.