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AC amplification achieve in natural electrochemical transistors for impedance-based single cell sensors


Impedance sensing of a dielectric microparticle

The primary goal of our work is to introduce a novel experiment to quantify the sensitivity of electrochemical impedance sensors operated in OECT or microelectrode configuration. To this finish we realized the experimental setup proven in Fig. 1a, b. The set-up comprises a dielectric microparticle (with diameter of fifty µm) hooked up to the underside a part of an AFM cantilever to have micrometric management of its place within the 3 spatial instructions (Fig. 1c). As soon as the microparticle is finely aligned with the x-y coordinates on the middle of the sensor floor, we use the z-stage of the microscope to regulate the particle-sample distance d. Contact of the microparticle with the sensor floor is set by the onset of a repulsive power appearing on the AFM cantilever. The electrical circuit to function the OECT impedance sensor comprises an Ag/AgCl wire that’s used because the gate electrode, controlling {the electrical} potential of the aqueous electrolyte resolution (0.1 M PBS). A DC voltage VD,DC is utilized between the supply (S) and drain (D) electrodes of the OECT to drive the digital present ID,DC within the PEDOT:PSS channel. The measured switch and output traits of a typical OECT (Fig. 1d), display that the gate voltage successfully modulates the channel present. (Your complete set of switch curves of the units with totally different channel dimensions used within the experiments is reported in Supplementary Fig. 1). For impedance sensing, we superimpose a small sinusoidal oscillation sign VG,AC (with amplitude 10 mV and angular frequency ω) on the gate bias VG,DC. This results in an AC present within the PEDOT:PSS layer47, which is measured by a lock-in amplifier linked to the supply contact (IS,AC).

Fig. 1: Impedance sensing of a dielectric microparticle.
figure 1

a, b Schematic of the experimental set-up in OECT and microelectrode configuration. c Optical microscopy picture of the microparticle hooked up to the AFM cantilever. d Optical picture of the energetic PEDOT:PSS channel (W × L = 200 × 50 μm) in an OECT system, whose DC switch and output traits utilizing a Ag/AgCl gate are proven within the inset. e Variation of the AC present amplitude in OECT and microelectrode configuration throughout repeated microparticle strategy and retract. Consecutive measurements are indicated with totally different colours. f Element on a single microparticle distance—AC present measurement acquired with an OECT and a microelectrode system. Totally different colours are used to point the microparticle strategy and elevate. The slope of the curves yields the sensitivity s of the impedance sensor.

Within the microelectrode configuration (Fig. 1b), the circuit is simplified, as supply and drain electrodes are in brief circuit and are collectively linked to the lock-in amplifier. Subsequently, no OECT channel present is current, and all the electrical present measured throughout impedance sensing is the gate present IG,AC, flowing from the electrolyte into the PEDOT:PSS layer. All different parts are equivalent to the OECT configuration to allow a direct comparability right here carried out at 5 totally different frequencies (0.12, 0.33, 1.17, 3.33 and 11.7 kHz).

In Fig. 1e, we present the outcomes of a typical microparticle distance—AC present experiment carried out at 1.17 kHz excitation frequency, which was chosen for instance. The amplitudes of the AC currents in OECT and microelectrode configuration are plotted as a operate of time. Through the experiment, the microparticle is approached and retracted from the system channel for 3 consecutive instances. For each configurations, the present amplitude follows the movement of the microparticle in a extremely reproducible method over consecutive cycles, highlighting the steadiness of the characterization technique. In Fig. 1f, the identical information are plotted as a operate of the gap between microparticle and sensor floor (normalized information are reported in Supplementary Fig. 4). Each varieties of units produce a reversible, linear response through which the strategy results in a discount in AC amplitude. Qualitatively, this response is anticipated, because the microsphere represents a barrier for the ionic present within the electrolyte: when it’s near the sensor floor, the half house via which ions can strategy the energetic layer is diminished, thus rising the efficient impedance of the electrolyte Zel. Consequently, upon strategy, the interfacial impedance measured with the sensor will increase and the AC present amplitude drops. We notice that in first order approximation an identical response is anticipated when a organic cell adheres to the sensor floor.

The outcomes are essential for our purpose as they enable the quantitative evaluation of the sensitivity of the impedance sensor. For the case of a excessive sensitivity, small adjustments within the impedance Zel trigger massive variation in AC present amplitude. Subsequently, we outline the sensitivity as s = ∂IAC/∂Zel. In our experiment, ∂Zel is immediately associated to the microparticle displacement ∂Zel = d. The proportionality fixed p quantifies how a lot the electrolyte impedance adjustments when the microparticle-channel distance d is modified. Subsequently, p is unbiased on the sensor configuration (OECT vs microelectrode) however is barely a measure of the variation of the electrolyte impedance when the geometry of the ionic present barrier is modified. We acquire its numerical worth for every channel geometry by becoming the microelectrode impedance spectrum (see Supplementary Fig. 5). Accordingly, the sensitivity is given by the slope of the strategy curves proven in Fig. 1f. For the channel geometry of W × L = 100 × 100 µm, we acquire values of sOECT = 0.059 ± 0.002 nA/Ω and sμE = 0.023 ± 0.006 nA/Ω. The values present a larger sensitivity within the OECT system with respect to the microelectrode, as a result of contribution of OECT channel present to the AC response.

Quantitative mannequin for PEDOT:PSS-based impedance sensors

We developed an analytical mannequin to precise the impedance sensitivity s as a operate of the sensor operation situations, materials properties and geometry. Goal is a quantitative understanding of the elements that enhance sensitivity in OECT configuration with respect to PEDOT:PSS microelectrodes. Many research decouple cost transport in OECTs in an digital and an ionic circuit14. A schematic of this illustration is reported in Fig. 2a, the place the parts of the digital and the ionic circuit are indicated in blue and orange, respectively. Digital cost carriers (holes) are pushed by the drain voltage VD,DC and carry the channel present in an OECT, whereas ionic cost carriers are pushed by the gate voltage VG = VG,DC + VG,AC and modulate the focus of holes and, consequently, the digital conductivity of the transistor channel. The restricted conductivity of the electrolyte in addition to the presence of dielectric objects near the sensor floor generate an impedance Zel, which causes a possible drop within the electrolyte, and the voltage on the electrolyte/channel interface ({V}_{{{{rm{G}}}}*,{{{{rm{AC}}}}}}) that acts on the channel and determines the drain present42. For the impedance sensing, we have an interest within the AC response of the transistor and we specific the AC present flowing within the OECT channel as ({I}_{{{{{rm{ch}}}}},{{{{rm{AC}}}}}}={g}_{{{{rm{m}}}}}cdot {V}_{{{{rm{G}}}}*,{{{{rm{AC}}}}}}). Following Bernards mannequin48, ({g}_{{{{{{rm{m}}}}}}}) may be expressed as ({g}_{{{{{{rm{m}}}}}}}^{{{{{{{rm{lin}}}}}}}}=-frac{W}{L}mu {c}_{{{{{{rm{v}}}}}}}t{V}_{{{{{{rm{D}}}}}},{{{{{{rm{DC}}}}}}}}) and ({g}_{{{{{{rm{m}}}}}}}^{{{{{{{rm{sat}}}}}}}}=-frac{W}{L}mu {c}_{{{{{{rm{v}}}}}}}t({V}_{{{{{{rm{G}}}}}},{{{{{{rm{DC}}}}}}}}-{V}_{{{{{{rm{t}}}}}}})) in linear or saturation situations. In these expressions, W, L, and t point out the width, size and thickness of the sensor channel, µp the holes mobility, cv the volumetric capacitance of PEDOT:PSS and Vt the OECT threshold voltage.

Fig. 2: Quantitative mannequin for PEDOT:PSS-based impedance sensors.
figure 2

a Equal circuit of an OECT throughout impedance sensing. Orange and blue colours point out ionic and digital elements of the circuit, respectively. b Modeling of the present frequency spectra of an OECT and a PEDOT:PSS microelectrode. c Normalized OECT supply present amplitudes as a operate of the utilized frequency for various channel geometries. d OECT sensitivity throughout the microparticle impedance sensing experiment. The error bars are obtained by averaging between the strategy and elevate curves of the AFM experiment. e Comparability between the sensitivity of an OECT and a microelectrode having the identical dimensions. f OECT achieve for various channel geometries.

To derive the general AC present, you will need to notice that in AC transport situations the supply (and drain) present indicators are composed of two contributions:

$${I}_{{{{rm{S}}}},{{{{rm{AC}}}}}}=,{I}_{{{{{rm{ch}}}}},{{{{rm{AC}}}}}},+,{f}_{{{{{rm{OECT}}}}}}cdot {I}_{{{{rm{G}}}},{{{{rm{AC}}}}}}$$


The primary (Ich,AC) originates from the channel present, whereas the second (IG,AC) is as a result of gate present and regards the capacitive present that has rising significance at greater frequencies. Its worth is given by IG,AC = VG,AC/ZG through which ZG = Zel + Zch is the general impedance of the sensor given by the sequence mixture of the electrolyte impedance Zel and the impedance associated to the PEDOT:PSS channel capacitance Zch = 1/(iωCch). The channel capacitance can additional be associated to the geometry and the volumetric capacitance of the PEDOT:PSS layer. Attainable contributions resulting from parasitic capacitances are uncared for for simplicity.

The issue fOECT in Eq. (1) determines how the gate present is distributed between the supply and the drain terminal49. Usually, the issue fOECT is dependent upon the bias situations (VD,DC and VG,DC), on channel geometry and on AC or DC measurement situations50. In our case, we think about the AC transport regime the place the gate present is a pure capacitive present with out Faradaic contributions. Additional, in our biasing situations (VG,DC = 0.1 V and VD,DC = −0.4 V) a big detrimental potential is utilized to the drain electrode resulting in a depletion of holes from the channel area close by the drain contact17. Consequently, the capacitive gate present encounters a resistive barrier on the drain electrode and as a substitute enters into the supply electrode. Because of this, we set fOECT = 1 in our information evaluation for every sensor geometry. The worth is supported by numerical becoming procedures of our frequency-dependent information resulting in values shut to at least one (see Supplementary Desk 1).

The determine of advantage of the OECT impedance sensor (the sensitivity sOECT) quantifies its functionality to transduce a variation of Zel right into a present output. This may be calculated from the mannequin by differentiating Eq. (1):

$${s}_{{{{{rm{OECT}}}}}}=,left| frac{partial ({I}_{{{{{rm{ch}}}}},{{{{rm{AC}}}}}}+{f}_{{{{{rm{OECT}}}}}}{I}_{{{{rm{G}}}},{{{{rm{AC}}}}}})}{partial {Z}_{{{{{rm{el}}}}}}}proper|=left|{s}_{{{{{rm{ch}}}}}}+{f}_{{{{{rm{OECT}}}}}}cdot {s}_{upmu {{{rm{E}}}}}proper |$$


After inserting the expressions for the 2 AC present contributions and differentiation, we acquire for the channel sensitivity



and the sensitivity of the microelectrode

$${s}_{upmu {{{rm{E}}}}}=frac{1}{{Z}_{{{{rm{G}}}}}^{2}}{V}_{{{{rm{G}}}},{{{{rm{AC}}}}}}$$


The specific mathematical expressions relating the units’ sensitivity to the utilized frequency are reported in Supplementary Dialogue 1. The suitability of this straightforward strategy to mannequin the AC response of an OECT is demonstrated in Fig. 2b, c. Determine 2b compares the frequency response of an OECT and of a microelectrode with the mannequin predictions. The PEDOT:PSS channel width and size are W = 100 μm and L = 100 μm, respectively. The channel capacitance and the electrolyte resistance Rel have been extracted for every system geometry by becoming the microelectrode impedance spectrum with an equal RC circuit. The common volumetric capacitance of PEDOT:PSS resulted to be cv = 28 ± 2 F/cm3, acquiring a end result according to literature findings20. The OECT system reveals a considerably greater present within the low frequency area. Right here the digital channel present Ich prevails, and the transistor demonstrates clear amplifying properties. Then, above a cutoff frequency fc = 625 Hz, the transistor response is restricted by the sluggish ionic transport between the channel and the electrolyte51. On the identical time the microelectrode’s response will increase with frequency till a present limitation is reached as a result of electrolyte impedance. As a consequence, within the excessive frequency restrict each impedance sensor configurations yield the identical present response. Importantly, the cutoff frequency that determines the OECT amplification is set by the channel geometry as demonstrated in Fig. 2c. The plot of the present amplitude versus frequency for OECTs with totally different channel sizes clearly reveals that with rising channel space and size, a powerful discount in fc is noticed.

Lastly, we systematically examine the OECTs sensitivity in direction of electrolyte impedance adjustments with the microsphere experiment launched above. Determine 2nd reveals the measured values for sOECT obtained for 3 totally different channel geometries at totally different AC frequencies. Equation (2) is in glorious settlement with the frequency dependence of the measured information, permitting to renew the principle findings of the mannequin with the next statements. (i) Within the low frequency regime, the OECT sensitivity reveals a linear enhance with frequency till it reaches a sensitivity most sOECTmax. On this frequency vary the sensitivity will increase strongly with the channel side ratio W/L as it’s extremely managed by the OECT transconductance. (ii) Within the intermediate regime, OECT impedance-based sensors have a most sensitivity at an outlined operation frequency, which corresponds to their low-pass cutoff fc. The spectral place of fc (see Supplementary Dialogue 2) is especially decided by the channel space, which defines the channel capacitance Cch and the electrolyte resistance Rel. Adjustments within the side ratio W/L modify the OECT transconductance and have a direct influence on the worth of sOECTmax. Accordingly, the frequency cutoffs of the 100 × 100 μm and the 200 × 50 μm constructions are virtually coincident, however the latter reveals the next sensitivity most. The smaller dimensions of the 50 × 50 μm system shift its fc in direction of greater frequency, whereas sOECTmax is restricted by the sq. side ratio. (iii) Within the excessive frequency restrict, the OECT sensitivity outcomes from the capacitive gate present and is usually managed by the realm of the channel. That is additionally evident from Fig. 2e, the place we evaluate the sensitivity of a microelectrode and an OECT with the identical dimensions (W × L = 200 × 50 µm). The transistor amplification, which is important at low frequencies, has a related influence on the sensitivity. Nevertheless, at excessive frequencies the response of each units is restricted by the electrolyte resistance and no important variations are current. Such an statement is mirrored by a frequency-dependent OECT achieve, which may be immediately calculated with our mannequin from Eqs. (2) and (4):

$${{{{{rm{achieve}}}}}}_{{{{{rm{OECT}}}}}}=,20cdot {{{{rm{log }}}}}_{10}left(left| frac{{s}_{{{{{rm{OECT}}}}}}}{{s}_{upmu {{{rm{E}}}}}}proper| proper)=20cdot {{{{rm{log }}}}}_{10}left(frac{{g}_{{{{rm{m}}}}}}{omega {C}_{{{{{rm{ch}}}}}}}+{f}_{{{{{rm{OECT}}}}}}proper)$$


The OECT achieve is highest within the low-frequency regime, however continues to be important within the 0.1–10 kHz vary, the place the impedance of the cell layers is usually measured52. This justifies using a transistor construction for high-precision bioelectronic impedance sensing experiments5. Determine 2f demonstrates that the OECT achieve is a geometry-dependent parameter. The smallest system (W × L = 50 × 50 µm) reveals the very best achieve, whereas an oblong channel geometry is preferable for OECTs with the identical space, because the transconductance will increase with the W/L ratio whereas the channel capacitance stays fixed.

Single-cell impedance sensor experiment

We demonstrated the worth of the mathematical mannequin right here proposed for the optimization of a PEDOT:PSS-based single-cell impedance sensor by monitoring single-cell adhesion and detachment in an in vitro experiment, concurrently measuring the impedance adjustments with each an OECT and a microelectrode. In response to the mannequin prediction and the AFM experiment, we patterned the system channels with a 200 × 50 µm rectangular geometry, which supplies the most effective performances by way of sensitivity. The T98G cell line cultured in Minimal Important Medium was diluted to have a closing density of 1 × 103 cells/cm3 and poured on the floor of the impedance sensors (see the “Strategies” part for full particulars). After seeding, the cells reached the underlying substrate by gravity. We microfabricated a linear array of 10 PEDOT:PSS channels (see Supplementary Fig. 6) to largely enhance the chance of a single-cell settling onto a sensor. An optical picture of the ultimate experimental configuration is reported in Fig. 3a, exhibiting a single T98G cell positioned on the middle of the PEDOT:PSS channel. The encapsulation of the metallic electrodes with detrimental photoresist insulates the system from all of the remaining cells which aren’t mendacity within the PEDOT:PSS energetic layer. We acquired the present spectra of each the OECT and the microelectrode at consecutive time intervals to make a real-time detection of the cell adhesion course of. To emphasize the total consistency of the measurements acquired with the OECT and the microelectrode, we plot in Fig. 3b the present amplitudes measured at 625 Hz as a operate of time. The sensing frequency was chosen in correspondence to the OECT cutoff (measured at time t = 0), the place the mannequin signifies the utmost sensitivity. The indicators acquired in the beginning of the experiment are secure round a most worth. Afterward, at time t = 20 min the present begin to lower, indicating the start of the cell adhesion course of. This produces a quickly various response till t = 60 min, when the lower turns into slower, and the present stabilizes round a minimal worth. At t = 200 min, we used a cell dissociation agent (trypsin) to fully take away the cell from the pattern floor, and units recovered their unique present amplitude.

Fig. 3: Single-cell impedance sensor experiment.
figure 3

a Optical picture acquired after seeding a single cell on the middle of a PEDOT:PSS sensing channel. b Time evolution of the cell adhesion monitored with an OECT and a microelectrode. c Present spectra acquired within the OECT (straight line) and microelectrode (dashed line) configuration earlier than and after trypsinization. d Experimental OECT and microelectrode sensitivity. Error bars are obtained by averaging between n = 4 measurements acquired at totally different time. e Experimental OECT achieve. Error bars are calculated from the experimental sensitivities by making use of Eq. 5.

We report in Fig. 3c the total present spectra acquired earlier than and after the therapy with trypsin (t = 180 min and t = 240 min, respectively). The cell adhesion produced a big shift of the OECT low-pass cutoff in direction of smaller frequency. In parallel, the present spectrum of a management system positioned in the identical reservoir, however with no cell seeded on the PEDOT:PSS layer, remained unaltered (see Supplementary Fig. 7). After trypsinization, the preliminary cutoff frequency is totally recovered. These mixed observations clearly display that the cutoff shift is barely brought on by the single-cell adhesion on the PEDOT:PSS layer. The identical issues may be prolonged to the PEDOT:PSS microelectrode. Right here, the cell adhesion course of is revealed by a lower within the gate present amplitude, which reaches its minimal at t = 180 min, coherently with the OECT measurements. After the cell detachment (t = 240 min), the present will increase to its unique values.

To supply a direct comparability between the sensing performances, we averaged the present amplitudes acquired when the cell is indifferent (t < 30 min and t = 240 min) and hooked up (120 < t < 180 min), and we subtracted the ensuing values to calculate the experimental sensitivity for each units. Repeating this evaluation in the entire frequency spectrum, we obtained the curves reported in Fig. 3d, that affirm the outcomes of the quantitative AFM experiments (Fig. 2e). The transistor amplification has a significative influence on the system sensitivity within the frequency vary between 102 and 104 Hz, with a peak at 625 Hz, akin to the OECT cutoff. Then again, when the modulation frequency is excessive, the OECT transconductance turns into negligible, and the transistor construction doesn’t provide substantial benefits with respect to a microelectrode. The OECT achieve (Fig. 3e) was calculated by making use of Eq. 5 and reaches a price of 20.2 ± 0.9 dB on the highest sensitivity level (625 Hz).

The frequency response of the OECT achieve is effectively described by our mannequin for each experiments, the only cell in addition to the dielectric particle detection. Nevertheless, the OECT present amplitude discount is far bigger for the case of the only cell though the cell physique has a diameter that’s smaller than the dielectric particle. The impact is attributed to the a lot bigger impedance enhance brought on by the cell adhering to the sensor floor. Glioblastoma tumor cells corresponding to T98G secrete massive quantities of laminin and glycoproteins to self-assemble the basement membrane beneath their mobile physique53. We hypothesize that the basement membrane spreads beneath the cell physique on high of the PEDOT:PSS channel and acts as a barrier rising considerably the impedance. Trypsin therapy removes the cell physique and dissolves additionally the basement membrane, making the impact reversible



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