Ultrasensitive bio-detection using single-electron effect

Abstract

Single-electron devices are capable of detecting changes of the electric field caused by the presence of one single electron in their environment. These devices are optimized to identify the material that is in close contact with them based on the material's internal charge distribution or dipole moment. As an important practical use, they present the possibility of detecting bacteria, viruses, or pathogens. However, their practical use is hampered by their nano-meter size, which is normally an order of magnitude smaller than that of detected species, their very complex fabrication techniques, their cryogenic operational temperature, and the problem of bringing the said species in contact with the single-electron structure. In this document, a large scaled room temperature single-electron structure is introduced, and its ability to distinguish liquids based on their internal dipole moments is demonstrated. The device is a Schottky junction made of PtSi, as the metal contact, and the walls and surfaces of the porous Si, as the semiconductor. The reverse bias current-voltage (IV) characteristic of this device is sensitive to 1 ppm change in the dipole moment of the liquid entering its pores. The simple fabrication, easy testing procedure, high sensitivity, and fast response can make this device an optimized testing kit to identify the given bacteria, viruses, or pathogens dissolved in liquids.

Keywords: Bacteria identification; Liquid dielectric constant; Pathogen dipole moment; Porous silicon; Single-electron effect; Virus identification.

Graphical abstract

Fig. 1

The apparatus, with which both the anodic etching and Pt electrodeposition are performed, is made of a Teflon cell with two separate parts. The bottom consists of a Cu disc, which makes contact with the bottom side of the Si sample. Si is pressed against this disc by the screws that push the top Teflon part down to the bottom. An O-ring is placed on the Si sample and the pressure by the top Teflon part prevents the etching solution from leaking outside. The solution is poured inside a funnel in the top cell and makes contact with the Si bordered by the O-ring. The funnel is large enough to allow ultraviolet light to reach Si from the top. Etching is performed while the whole set up is suspended in an ultrasonic bath. Immediately after etching, the solution is washed out and a new solution for the electrodeposition of Pt is poured in. The electrodeposition is performed in the same cell.

Fig. 2

(a) SEM micrograph of a typical PtSi/Porous Si sample. The white areas at the walls of the pores are PtSi. As it can be seen, the pore openings are a few μm wide and their depth is about 10 μm. The porosity at the scale of the device is uniform and reproducible as witnessed in reproducible IV characteristics. Liquid enters the pores and makes contact with the PtSi covering the sidewalls. The PtSi layer creates the bright white areas seen in the graph. (b) The porous samples of which results are provided in this paper. Porosity is such that pores with a diameter of 10 μm are surrounding pores similarly to Fig. 1a. (c) The depth of the large pores reach down to 70 μm. This porosity has been used to help liquids with higher surface tension to enter the pores as well.

Fig. 3

The whole device and measurement set up is presented here. The sample and its connections are shown schematically here because the actual unit, given in Fig. b, is opaque and the details cannot be seen. Two printed circuit boards push on an O-ring and sandwich the sample. Only the porous area is exposed to the liquid that is poured into the opening. The O-ring prevents the liquid from reaching other parts of the sample. Connections to the porous area and Si itself happen by pressing Ag terminals on the contacts to these areas via Silver paste. The sample holder is inserted into the measuring circuit that sends current into the device and measures its voltage. The result is sent to a computer of which the USB port is used to power the circuit itself.

Fig. 4

Room-temperature reverse bias I–V curves of a PtSi/Porous Si junction in the air and when it is in contact with methanol. There is a remarkable difference in the IV curves. The threshold voltage for the single-electron transfer can be estimated by drawing two asymptotes to the curves, one at low and one at high voltages. Their intersection is a rough indication of the threshold voltage. In this case, the threshold voltage of the air has changed from −20.5 V to about −17 V for methanol. Notice that at the voltage of about −20 V the current has increased by about 0.3 mA from the air to methanol.

Fig. 5

DI water has a relative permittivity as about twice as methanol does but its IV curve does not change as much compared to that of methanol. The threshold voltage has decreased only by about half a volt compared to the threshold voltage of methanol changed by about 3.5 V as compared to the air (Fig. 4). This is due to the surface tension of water, which prevents it from completely entering the pores.

Fig. 6

The IV curves of DI water when a relatively nonpolar oil has been added to change its surface tension while decreasing its permittivity only slightly. As it can be seen, by increasing the oil concentration the IV curve moves to left indicating the effect of DI water is felt more by the device. Put differently, water has been able to enter the pores and change the IV curve although the permittivity of the solution has decreased by the addition of the oil. The IV curve when only oil is poured on the sample does not change much compared to other liquids.

Fig. 7

This diagram illustrates IV curves for the air and some alcohols of different permittivity such as methanol, ethanol, propan-2-ol, and ethyl acetate. The change in the IV curve follows the change in their dipole moments. The point to notice is the change in slope after the threshold voltage. The change in the slope is due to other factors besides the dipole moment that later can be used to better identify a liquid.

Fig. 8

These results demonstrate that a continuous change in the dipole moment creates a corresponding continuous change in the IV curve. Methanol was added to ethanol in very small amounts to continuously change its dipole moment. A corresponding change in the IV curve was observed. The limit is the noise of the circuit. Some representative graphs are provided in this figure. A noise of 60 nA in the circuit results in the detection of a 1 ppm change in the relative permittivity.