key: cord-0809776-bbk6bmu6
authors: Ashoori, Shiva; Raissi, Farshid; Naderpour, Maryam; Ghezelayagh, Mohammad Mahdi; Zadeh, Reza Malekabadi
title: Ultra sensitive bio-detection using single-electron effect
date: 2020-10-14
journal: Talanta
DOI: 10.1016/j.talanta.2020.121769
sha: c19d622eb9ca23a214f1c0380037f3cd16e1ca70
doc_id: 809776
cord_uid: bbk6bmu6

Nanosized particles can detect minute changes in surrounding electric fields via a phenomenon known as single-electron effect. Sensitivity can reach as much as counting single electrons. This phenomenon promises the possibilty of detection of atoms, molecules, cells or pathogens passing over or suspended around the nanosized particles whose IV characteristics is being monitored. Unfortunately, practical realization of such devices has been challenging because of their small size and low operating temperature, well below freezing temperature of regular liquids. Here we introduce PtSi/porous Si Schottky junction as a room temperature large size single-electron device capable of detecting very minute changes in the electric fields and hence identifying molecules, cells, and bacteria suspended or dissolved in liquid solutions. Encouraging results for detection of healthy and cancerous human cells as well as liquids and microorganisms are provided.

The importance of the fast and reliable detection of biological agents such as cancerous 47

Since its discovery, the porous Si has been the subject of much research because of its 80 large surface area as well as its unprecedented electrical [9-10], optical [11] [12] , and 81 mechanical [13] [14] properties and also its compatibility with silicon-based microelectronic 82 devices [15] . Some of its applications include being used in areas such as optoelectronics 83

[16], micromachines (as sacrificial layers) [ the electric fields, created by the sharp and needle-like pores, help to produce a single-86 electron effect. Platinum silicide (PtSi) is used as the metal contact because of its ease of 87 formation and its well-characterization, reproducibility, and uniformity. 88

There are three practical problems with the realization of the single-electron device as 89 pathogen detector. First, it is the size of the island, which is less than the sizes of the cells, 90 viruses, and bacteria. This creates a problem because their alignment, with respect to the 91 island, affects the measurements unpredictably. Second, the threshold voltage is normally 92 less than 100 mV, and as a result, they need to be cooled to cryogenic temperatures to 93 overcome thermal noises. Third, the detected species must be brought to close proximity of 94 the island, which can only be accomplished in a liquid solution. Liquid solutions, however, 95 can short out the two connections. 96

In this paper, experimental results for the realization of a large scaled single-electron liquid 97 detector operating at the room temperature are presented. It is demonstrated that it can 98 distinguish and identify various liquids based on their dipole moments [29] . It is shown that 99 by changing the dipole moment, the IV characteristics changes consistently as well. The interesting consequence, which makes this junction reproducible, is the fact that PtSi is 125 created by the diffusion of Pt into the Si layer. The electrical junction between PtSi and Si 126 occurs inside and below the initial Si surface, and as a result, is not affected by the impurities 127 or surface states of the Si top layer. 128

After the creation of PtSi, the whole structure is put in an etching solution to completely 129 empty the pores from the remaining Pt. This process takes about 5 minutes. Connections to 130

Si as well as PtSi, which has covered the porous area, are made via a silver paste. Silver paste 131 creates an ohmic contact to Si, and since PtSi is a metal alloy, a low loss connection is created 132 on it as well. The contact resistance to both PtSi and Si was measured via a four-point probe 133

and was estimated to be below 1 mΩ [8, 33] . 134

Conditions given in Table 1 level was proven to exhibit the single-electron effect at the room temperature [8] and also this 139 structure has been successfully tested as a gas and IR detector based on the same underlying are identical in the fact that the material filling the pores changes the IV curve based on its 142 dipole moment. Such porosity level exhibits a threshold voltage of about -10 V [8] , [33] [34] [35] . 143

For the liquid detection, it might be more appropriate to create pores with larger openings to 144 help liquids enter the pores. The porous samples shown in Fig. 2b The porous area is about 2 cm 2 , and the O-ring covers its inside borders exposing only 162 about 1 cm 2 of the porous surface to the liquid. The O-ring prevents the leakage and 163 penetration of liquid underneath and into the Si side surrounding the porous area. As it was 164 mentioned before, only the reverse bias IV curve exhibits the single-electron effect, which 165 manifests itself as a relatively small current, at the voltage values below the threshold voltage, 166 but it increases drastically afterwards. The relatively small current below the threshold 167 voltage is also an order of magnitude less than what is observed in regular PtSi/Si Schottky 168 junctions of the same size [8, 34] . Regular samples not only exhibit remarkably larger 169 currents in reverse bias, but also are not sensitive to IR, gases [8, 34, 36] , and liquids. 170 structure, and then integrating it over the distance to infinity. This provides the energy 174 necessary for transferring an electron to the structure, which is the inverse of capacitance. 175

Consequently, a larger electric field around a structure should result in a larger voltage and a 176 smaller capacitance. The sharp edges of the porous surface can be likened to needle-like 177 structures. As it is known, the electric field around sharp objects such as needles can increase 178 by an order of magnitude compared to the fields created by the same charges over a plane 179 surface. These large electric fields are exactly the reason why porous surfaces provide the 180 capacitance reduced by an order of magnitude. They are also responsible for creating a very 181 narrow Schottky barrier allowing electrons to tunnel from PtSi into Si. In effect, electrons do 182 not see the Schottky potential barrier, and the only barrier they have to surmount is the single-183 electron barrier, which is manifested as the threshold voltage. 184

There are two equivalent ways of explaining the gas and liquid detection by the 185

PtSi/porous Si junction. One is to consider that molecules, which come in contact with PtSi critical because, as will be discussed later, the entire IV curve is used to identify a liquid. 217

The difference between these two graphs is quite clear and demonstrates that this device 218 can distinguish liquids unambiguously. Methanol has the largest relative permittivity 219 amongst the alcohols used in experiments and has a value of 33 [37]. The relative 220 permittivity, dielectric constant, and dipole moment reflect the same electrical characteristics. 221

The relative permittivity or dielectric constant is used for solids and liquids, but when talking 222 about individual molecules and atoms the dipole moment is used. It is dipole moments of 223 molecules and their alignment that give rise to the specific relative permittivity or dielectric 224 constant of a solid. 225

Although the dipole moment has the highest effect on the IV curve, there are other 226 parameters that affect the IV curve as well. As it is seen in Fig. 4 and as will become 227 apparent in subsequent figures, a liquid not only changes the threshold voltage, but also 228 changes the slope of the IV curve after the threshold voltage. In experiments with the gas 229 detection, a change in the gas content never changes the slope of the IV curve after the 230 threshold [8, 33, 36] . Furthermore, while a decrease in the threshold voltage must be 231 accompanied by a consistent increase in current at all voltage values, liquids do not follow 232 this trend. For some of them, the current decreases at voltage values below half the threshold 233 voltage. The change of slope and the decrease of the current at a low voltage are assumed to 234 be related to the liquid's electron affinity and its electrical resistance but are subject to 235 investigation. These two effects are not covered here, but even though they seem to it would be conceivable that two liquids having different constituents would have the same 239 dipole moment. But two liquids are less likely to be the same in dipole moment, resistivity 240 and electron affinity. This also means that the entirety of a liquid's IV curve could be used as 241 its fingerprint. 242

On the other hand, the surface tension can completely override the effects of the dipole 243 moment by preventing the liquid from entering the pores. DI water has a relative permittivity 244 twice as much as methanol does, but it does not decrease the threshold voltage as it is 245 supposed to. Fig. 5 shows the corresponding IV curve of DI water and a device in the air. 246

The change in threshold voltage is very small compared to that of methanol. To see whether 247 the surface tension is actually playing a role here, a relatively non-polar oil with the dielectric 248 constant of 3 was added to DI water to change its surface tension without much affecting its 249 permittivity. The result for various concentrations of the oil is provided in Fig. 6 . The oil by 250 itself affects the IV curve much less than DI water does. As the oil is added to DI water, the 251 solution's dipole moment decreases, and the threshold voltage should increase. But IV curve 252 changes in the opposite direction, and its threshold voltage decreases. It indicates that the oil 253 is helping DI water enter the pores and more affect the IV curve by decreasing the surface 254 tension. 255

In Fig. 7 results for alcohols with different dipole moments are shown. Each result has a 256 different IV curve based on its dipole moment. The distinguishing factor between them is not 257 just the threshold voltage, but also the plurality of their IV curves; therefore, obtaining the 258 exact threshold voltage is not needed for the practical usage. The threshold voltage is used as 259 a rough estimate for the analytical reasoning and to test existing theories. The porous samples of which results are provided in this paper. Porosity is such that pores J o u r n a l P r e -p r o o f 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 Oring 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. 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. 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 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. 

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