Homocysteine Solution-Induced Response: Comparison
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Homocysteine (Hcy) is a non-protein, sulfur-containing amino acid, which is recognized as a possible risk factor for coronary artery and other pathologies when its levels in the blood exceed the normal range of between 5 and 12 μmol/L (hyperhomocysteinemia). At present, standard procedures in laboratory medicine, such as high-performance liquid chromatography (HPLC), are commonly employed for the quantitation of total Hcy (tHcy), i.e., the sum of the protein-bound (oxidized) and free (homocystine plus reduced Hcy) forms, in biological fluids (particularly, serum or plasma). Here, the response of Aerosol Jet-printed organic electrochemical transistors (OECTs), in the presence of either reduced (free) and oxidized Hcy-based solutions, was analyzed. Two different experimental protocols were followed to this end: the former consisting of gold (Au) electrodes’ biothiol-induced thiolation, while the latter simply used bare platinum (Pt) electrodes. Electrochemical impedance spectroscopy (EIS) analysis was performed both to validate the gold thiolation protocol and to gain insights into the reduced Hcy sensing mechanism by the Au-gated OECTs, which provided a final limit of detection (LoD) of 80 nM. For the OECT response based on Platinum gate electrodes, on the other hand, a LoD of 180 nM was found in the presence of albumin-bound Hcy, with this being the most abundant oxidized Hcy-form (i.e., the protein-bound form) in physiological fluids. Despite the lack of any biochemical functionalization supporting the response selectivity, the findings discussed in this work highlight the potential role of OECT in the development of low-cost point-of-care (POC) electronic platforms that are suitable for the evaluation, in humans, of Hcy levels within the physiological range and in cases of hyperhomocysteinemia.

  • homocysteine
  • organic electrochemical transistors
  • point of care testing
  • cardiovascular risk

1. Introduction

In recent years, the rising interest in Bioelectronics has mainly been supported by the astonishing development of new electronic devices based on conducting organics (i.e., conjugated polymers). In several cases, indeed, these compounds are able to exhibit a mixed ionic-electronic conduction mode, which makes them particularly effective for their proper interfacing/interaction with biosystems [1].
A number of organic electronic devices in a transistor-like configuration, such as EGOFETs and OECTs, have been shown to be effective for the detection of biomolecules in biosystems through user-friendly and cost-effective approaches [2][3]. In particular, OECTs have emerged as versatile tools that are able to implement a Lab on Chip approach (allowing, for instance, the monitoring of biomolecules’ properties [4] and the properties of cells’ physiology [5], or even of neuromorphic function [6]). Such devices are able to convert changes in bioanalyte concentrations into measurable electronic current variations, preserving sufficient accuracy, reproducibility and sensitivity. In particular, the OECT standard architecture consists of a PEDOT:PSS (poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate) channel, filling the gap between two metal electrodes (source and drain), which is interfaced to an electrolyte containing the third electrode, named the gate. OECTs can work in a liquid medium for prolonged time at operating voltages well below 1 V. These devices implement an ion-to-electron transduction with a marked amplifying capability, since, during their operation, the ionic species in the electrolyte are reversibly forced, by the gate voltage, towards the PEDOT:PSS channel, producing its charge de-doping and the related decrease in conductivity. The surfaces of the main OECT components, especially the gate, can be bio-functionalized to improve the final selectivity towards the desired analyte (e.g., glucose, dopamine, or DNA) via specific electrochemical or biological interactions [7]. The combination of all these features has contributed to the rising interest in this sensing platform for use in potential diagnostic tools in laboratory medicine or as POC testing devices. Although the adoption of functionalization protocols is actually a crucial step for the operation in real bio-organic fluids, such as serum/plasma, urine and saliva, it is, however, possible to design strategies targeted to tailoring a proper sensing operation where, for example, false negative results are minimized upon the choice of suitable electrodes (to be eventually functionalized) being inert towards different interfering species [8]. More generally, the experimental design may benefit from the knowledge of the main features of both the biologic matrix [9] and the bioanalyte [10] to be detected by the OECT biosensor. In this respect, the features of some molecules that are of great interest in medicine may help to establish simple and cost-effective methods aimed at studying their physiochemical characteristics, while offering a tool for their determination in physiological environments.
Homocysteine (Hcy), a non-protein, sulfur-containing amino acid (see chemical structure reported in Figure 1) is the demethylated methionine derivative, occurring in the methionine-homocysteine cycle. Although, as an amino acid, Hcy is endowed with the presence of both one amino and one carboxyl group, it is not utilized in protein biosynthesis because it is not encoded by any codon-anticodon system. It is important to mention that, due to its high pKa, the free sulfidryl group on the Hcy side chain is easily oxidized at a physiological pH [11]. Hence, because of the strong reactivity of this thiol group, only about 1 ÷ 2% of Homocysteine is usually present in its free reduced form. Conversely, most of Hcy is carried in circulation in the form of a heterodimer that is mostly bound to the cysteine of serum albumin, while only a minimum is in the homodimer protein-free form (homocystine). Homocystine-mixed disulfides with free homocysteine or cysteine represent less than 15% of the total Hcy in serum. The Hcy-protein covalent disulfide adduct is by far the most abundant (for instance, at least 80% of homocysteine is bound to albumin) (Figure 1).
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Figure 1. Various forms of Homocysteine present in vivo. The albumin adduct oxidized derivative (protein bound) is, by far, the most prevalent, while the disulfide is normally almost negligible, except under some pathological conditions (e.g., homocystinuria). Although some authors hypothesized a role for either the protein-bound oxidized or the free forms, in clinical practice, total homocysteine is detected (tHcy), i.e., the sum of all forms, since current laboratory methods have been set up by accomplishing a preliminary reducing step (red arrows) prior to analytical procedures for quantitation.
Normal levels of tHcy in serum range between 5 and 10–12 μmol/L (for female and male subjects, respectively), while hyperhomocysteinemia (HHcy) is an excess of Hcy, normally classified as mild (up to 15 μmol/L), moderate (16–30 μmol/L), intermediate (31–100 μmol/L), and severe (>100 μmol/L) [12][13][14]. HHcy is generally recognized as an independent cardiovascular risk factor for increased arterial and venous thrombophilic states [15], as well as a number of other pathologies [16]. The importance of Hcy is related to cardiovascular risk in general, but especially in hypertensive individuals, in whom folate supplementation may be an important measure to be explored [17][18].
Hcy quantitation in serum or plasma matrices is a part of the thrombophilic risk profile evaluation, and is generally achieved using high-performance liquid chromatography (HPLC) or immunometric routine methods [19]. More recently, optical and electrochemical methods were proposed, although they have not yet been introduced in routine practice [20].

2. Electrochemical Impedance Spectroscopy (EIS) Experiments on Thiolated Gold SPE

A comparative EIS analysis, carried out by acquiring the Nyquist plot, i.e., the imaginary part of the complex impedance (Z(ω) = Z′ + iZ″) as a function of its real part, for a bare (black symbols in Figure 2a) and an incubated gold electrode (red symbols in Figure 2a), was conducted with the aim of showing that the gold incubation in rHcy solutions promotes the thiolation of the electrode. In particular, EIS measurements were performed in a physiological electrolyte (1× PBS, pH 7.3) upon incubation of the Au working electrode of a screen-printed electrode (SPE) by a Hcy/HCl/PBS solution. For this experiment, the concentration of Hcy was fixed at 10 µM, i.e., in the middle of the investigated Hcy concentration range.
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Figure 2. (a) Nyquist plot for the gold working electrodes of a SPE before (black symbols) and after (red symbols) their incubation in Hcy:HCl:PBS solutions (HCy concentration of 10 µM); continuous lines are the related fitting curves for the proposed equivalent circuit analysis; the inset shows a real picture of the commercial SPE used in this experiment. (b) Complex capacitance plot of the bare (black symbols) and covered (red symbols) electrodes calculated from the acquired complex impedances.
The acquired curves show specific differences both in their shape and magnitude, suggesting that the incubation process and the related electrode coverage were efficiently carried out. This occurrence was further investigated by an equivalent circuit analysis, where proper models (see the equivalent circuits reported in the inset of Figure 3a) were used to characterize the electrochemical properties of electrolyte/interface pairs, as well as the features of the surface coverages.
The whole set of the extracted fitting parameters is reported in Table 1. While, as expected, Rel (related to the electrolyte conductivity) is comparable in both cases and shows a magnitude of few tens of Ohm, the analysis of the other parameters provides evidence of the expected thiolation.
Table 1. Fit parameters extracted from EIS equivalent circuit model analysis.
Fit Parameters SPE Au Bare SPE Hcy-Functionalized
Rel (Ω) 30.6 32.4
Rct (MΩ) 1.2 4.16
CPE (Sxsn) 5.9 -
Ideality factor, n 0.944 0.978
CPEcoverage (Sxsn) - 1.68
Relpores (KΩ) - 511
CDL (nF) - 330
The assessed ideality factor (n) of 0.944 for the uncovered gold electrode indicates the formation of a double layer at the gold/PBS interface. On the other hand, the elevated Rct value (in the order of 1 MΩ) is fully compatible with the fact that, at the interface between a polarizable electrode and a saline medium, charge transfer phenomena are minimal, being due only to residual impurities between grain boundaries. In addition, the complex capacitance plot, calculated from the acquired complex impedance Z as C = 1/jωZ, is reported in Figure 3b.

3. Au-Gated OECTs

Providing that EIS measurements corroborate the efficiency of the thiolation protocol, we tested the gold electrode decoration upon SH-Au interaction as a probe for the detection of free rHcy by OECTs. To this end, gold wires were immersed for 24 h in Hcy:HCl:PBS solutions at different concentrations of both Hcy and HCl, leaving a ratio of 1:1 between Hcy and HCl.
Transfer curves, reported in Figure 3a, were recorded using PBS as an electrolyte for OECTs gated by gold electrodes that were previously incubated at different rHcy concentrations. The investigated Hcy range, from 100 nM to 1 mM, was selected in such a way as to comprise concentrations below the physiological range and above that of severe HHcy. According to our analysis, we found that the OECT response is more efficient (i.se. larger IDS modulation values) as the rHcy concentration in the incubation solution increases. An enhancement of the gate current values on Au electrode incubation was also observed.
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Figure 3. (a) (solid lines) OECT transfer curves measured after the gate gold electrode incubation in Hcy:HCl:PBS solutions with progressively-increased Hcy concentrations (from 100 nM (black curve) to 1 mM (magenta curve); the dotted orange curve was measured at the end of this set of measurements and using a gold electrode incubated at 1µM of Hcy. (b) The IDS modulation parameter, defined as [(IDS(@VGS = 0.8 V) − (IDS(@VGS = 0.1 V)]/(IDS(@VGS = 0.1 V), extracted from all the recorded transfer curves in panel (a), as a function of the Hcy concentration in Hcy:HCl:PBS solutions.
As reported in Figure 3b, the OECT sensing response can be described by a linear-log plot of the modulation parameter ΔIds as a function of rHcy concentration in Hcy:HCl:PBS solutions (calibration curve). The analyzed Hcy concentration range fell within the sensor dynamic range, except for the highest Hcy concentration of 1 mM. Therefore, from a linear regression of the calibration curve, it was possible to define the sensor LoD (i.e., the lowest measurable analyte concentration that could be detected in sample) with a high confidence level. In detail, following the standard rule by IUPAC, the LoD is defined as follows:
(1) LoD = 3σ/S
where σ is the standard deviation of the response (i.e., the standard deviation of the y-intercept of the regression line) and S is the slope of the regression line.

4. Pt-Gated OECTs

Hcy-based solutions, consisting of Hcy dissolved at various concentrations (from 100 nM to 1 mM) in a physiologic-like microenvironment made of a PBS:BSA (10 mM:600 µM), were used as gate electrolytes for Pt-gated OECTs. Since the neutral electrolytic environment favors the reactivity of the thiol group, oxidative reactions between albumin and Hcy were expected to promote Hcy–BSA binding to a large extent and in a rapid manner [21]. Therefore, the Pt-based OECTs operated in a simil-physiological microenvironment made of some free Hcy and an excess of albumin (BSA–Hcy). The resulting transfer curves are reported in Figure 4a. Interestingly, increasing levels of BSA-Hcy in the electrolytic solution were found to promote an enhanced IDS current modulation.
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Figure 4. (a) OECT transfer curves measured with a platinum electrode in albumin-based solutions (at a fixed BSA concentration of 600 µM) at different Hcy concentrations (from 100 nM to 1 mM). (b) Lin-log plot of the modulation parameter ΔIDS, extracted from all the recorded transfer curves in panel (a), as a function of the Hcy concentration in BSA:PBS solutions. The dashed blue line represents the modulation parameter for the blank measurement (BSA in PBS).
The corresponding calibration curve, consisting of the modulation parameter as a function of the Hcy concentration, is reported in Figure 4b. The nominal LoD by Pt-gated OECT, calculated from the calibration curve using Equation (1), was 180 nM. This value again represents the upper limit for the actual free Hcy concentrations, since a non-negligible fraction of Hcy is at least expected to be involved in the formation of albumin-aggregated forms. It is worth noting that the low modulation parameter assessed in the case of bare BSA-based electrolyte indicated that Hcy albumin-aggregate forms and eventual free Hcy fractions, not involved in the formation of Hcy-albumin aggregates, may be unambiguously detected by OECTs. Finally, for the sake of completeness, platinum-gated OECT were also investigated while using directly rHcy:HCl:PBS solutions (i.e., those used for gold thiolation) as electrolyte. In this case, data analysis indicates that this acidic electrolyte environment produces a remarkable increase in the LoD by one order of magnitude (LoD = 2.5 µM) in comparison with the Pt-gated OECT investigated in Hcy:BSA:PBS electrolytic solutions. A reduced dynamic range, represented by the linear range in the lin-log plot of the sensor response, was also observed.

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