Reagentless Detection of Low-Molecular-Weight Triamterene Using

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Reagentless Detection of Low-Molecular-Weight Triamterene Using Self-Doped TiO2 Nanotubes Felipe F Hudari, Guilherme G Bessegato, Flavio Cesar Bedatty Fernandes, Maria Valnice Boldrin Zanoni, and Paulo Roberto Bueno Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b01501 • Publication Date (Web): 16 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018

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Analytical Chemistry

Reagentless Detection of Low-Molecular-Weight Triamterene Using Self-Doped TiO2 Nanotubes Felipe F. Hudaria, Guilherme G. Bessegatoa, Flávio C. Bedatty Fernandesa, Maria V. B. Zanoni *, Paulo R. Bueno*

São Paulo State University (Unesp), Institute of Chemistry, Araraquara, São Paulo, Brazil

*Corresponding authors: [email protected]; [email protected] tel: +55 16 3301 9642, fax: +55 16 3322 2308, Instituto de Química, Universidade Estadual Paulista, Rua Prof. Francisco Degni, 55, 14800-060 - Araraquara, São Paulo, Brazil a These authors contributed equally.

ABSTRACT TiO2 nanotube electrodes were self-doped by electrochemical cathodic polarization, potentially converting Ti4+ into Ti3+, and thereby increasing both the normalized conductance and capacitance of the electrodes. One-hundred (from 1.9 ± 0.1 mF cm−2 to 19.2 ± 0.1 µF cm−2) and two-fold (from ∼6.2 to ∼14.4 mS cm−2) concomitant increases in capacitance and conductance, respectively, were achieved in self-doped TiO2 nanotubes; this was compared with the results for their undoped counterparts. The increases in the capacitance and conductance indicate that the Ti3+ states enhance the density of the electronic states; this is attributed to an existing relationship between the conductance and capacitance for nanoscale structures built on macroscopic electrodes. The ratio between the conductance and capacitance was used to detect and quantify, in a reagentless manner, the triamterene (TRT) diuretic by designing an appropriate doping level of TiO2 nanotubes. The sensitivity was improved when using immittance spectroscopy1,2 (2.4 × 106 % decade−1) compared to cyclic voltammetry (5.8 × 105 % decade−1). Furthermore, a higher linear range from 0.5 to 100 µmol L−1 (5.0 to 100 µmol L−1 for cyclic voltammetry measurements) and a lower limit-ofdetection of approximately 0.2 µmol L−1 were achieved by using immittance function methodology (better than the 4.1 µmol L−1 obtained by using cyclic voltammetry).

Keywords: Impedance-derived analytical methods, blue TiO2, titanium dioxide nanotubes, molecular gating, diuretic triamterene, electrochemical capacitance.

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INTRODUCTION There has been increasing interest in developing suitable nanoscale electro-active materials as electrochemical sensors aimed at medical,3,4 environmental,5 and industrial food applications.6 Changes associated with reduction/oxidation reactions upon the modification of an electrode can be used to detect an analyte through changes in electric current or the electrode’s surface potential. The oxidation/reduction reactions occur at specific potentials, allowing analytes to be differentiated solely based on the peak potential characteristically associated with the detection of the analyte.4 Indeed, this is achieved through the detection of an electrical current variation at the selected peak potential using cyclic voltammetry (CV),3,7 differential pulse voltammetry,4,5 or amperometry methods.5,8 Although they are sensitive, these methods are restricted to providing information regarding the sensorial mechanism. Alternatively, methods based on time-dependent spectroscopic approaches9–11 are inexpensive, portable, multiplexable, and label-free. Among them, time-dependent and impedance-derived spectroscopic approaches are powerful methods that can be optimised in terms of their electro-analytical performance, e.g. the time of measurements can be greatly increased by using the single-frequency mode. In principle, this approach allows the optimisation of any electro-active modified conductive interface for electro-analytical purposes.1,2,12,13 The use of nanoscale materials that exhibit distinctive electric properties offers the ability to perform suitable surface engineering to modify electrodes. These can be used to obtain higher sensitivity, faster signal response, lower fabrication cost, and miniaturisation, and can be produced using micro-fabrication facilities.5 Accordingly, redox polymers,3 carbon nanotubes,6 and graphene4,5,14 have been efficiently used as modified electrode materials for the detection of organic molecules, such as ascorbic acid,4,15,16 dopamine,4,17,18 uric acid,4,19 cytochrome c,7 and β-nicotinamide adenine dinucleotide (NADH)15. The appropriate modification of the different electrodes enables multiplexing to detect multiple targets.4 The use of carbon-based nanomaterials has allowed for a high sensitivity to detect small molecules, with significant advantages such as chemical inertness, large potential window range, low background current, and multiplexability.4,5,7,14 However, despite these qualities, the use of nanostructured carbon-based materials involves laborious and expensive fabrication processes, and difficulties with chemical control and reproducibility. Alternatively, electrodes made of highly ordered TiO2 nanotubes (TNTs) as n-type semiconductors, have emerged as a viable alternative owing to the facile modification of Ti metallic plates via electrochemical anodisation (see scheme 1).20 This yields TNT-modified electrodes of suitable electronic conductivity and high surface area. TNT-based electrodes have thus been successfully applied for the photo(electro)catalytic oxidation of contaminants, dye-sensitised solar cells,21 water splitting,22 and photo-electrochemical sensors.23 TNT electrodes can act as cathodes when not radioactively activated by light (irradiation of λ < 387 nm over anatase TNT form).24–26 Previous studies have demonstrated that TNT-based electrodes can be subsequently modified with metallic nanoparticles, such as Ag, Pt,27,28 Ni,29 Pd, and Pt/Au.30 Alternatively, they can also be modified with metallic-like conductive compounds, such as graphene oxide31 or reduced graphene oxide,31 and thus converted into anodes. Modified TNT electrodes with anodic characteristics have been used in supercapacitor devices,32–36 electrochemical oxidative cells,33,35,37,38 cleansing electrochemical systems,38–40 photo-anodes for water splitting cells,41 and as anodes in lithium ion batteries.42 In this work, we employed TNT electrodes with an anodic design, which were obtained by cathodic polarization of bare TNT-modified electrodes, thus named self-doped TNT (SD-TNT). We used impedance spectroscopy to study the physical and chemical electric characteristics before and after modification of the TNT electrodes. Interestingly, SD-TNT combines high conductance and capacitance (electrochemical charge


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capability) that are, from a classical point of view, two competing electric characteristics. SD-TNT has been used as a modified electrode with the aim of detecting small analytes, such as p-phenylenediamine and Fe(SCN)64−, and has demonstrated excellent analytical figures-of-merit compared to bare glassy carbon electrodes.26

Scheme 1. a) Schematic representation of a self-doped TiO2 nanotube grown on a titanium metal plate embedded in an electrolyte. b) SEM micrograph of self-doped TiO2 as synthesised in the present work.

SD-TNT structures are believed to possess Ti4O7 cluster defects26,36,43,44 which are rich in Ti3+ and thus modulate the conductivity of polarized TNTs. The cathodic polarization of TNT electrodes is performed under a negative potential, usually lower than −1.0 V; this generates oxygen vacancies associated with Ti4O7 clusters and Ti3+/Ti4+ mixed valences.32 This doping electrochemical process involves charge compensation through the intercalation of protons [Ti4+O2 + e− + H+ Ti3+(O)(OH)] (it is estimated that up to ca. 1% of the Ti4+ can be reduced to Ti3+),32 inducing a metallic-like behaviour36 within a concomitant, intrinsic, and proportionally higher capacitive behaviour (of the order of mF cm−2). The latter characteristics provide an unusually high electrochemical activity for the SD-TNT, studied using CV22,26,37,38 and was considered in this paper in more detail using impedance-based spectroscopic techniques. This paper reports, for the first time, the use of SD-TNT to monitor the diuretic triamterene (TRT). TRT is otherwise widely used for therapeutic purposes; to treat hypertension, cirrhosis, pulmonary disease, heart attack prevention, and disfunctionalities.45,46 However, owing to TRT’s diuretic characteristics, its use by athletes is prohibited as it prevents the detection of illicit substances during anti-doping surveillance.46 Accordingly, the consumption of TRT by athletes is controlled by the World Anti-Doping Agency (WADA). The purpose of the present work is to clarify the origin of the electrochemical characteristics of SD-TNT and demonstrate the use of these characteristics to quantify and selectively detect small molecules such as TRT. TRT concentrations were monitored in SD-TNT electrodes by using impedance-derived spectroscopic methods, which demonstrate enhanced analytical sensitivity as compared to, for instance, CV methods. Superior analytical characteristics were achieved because TRT can be oxidised in the presence of SD-TNT at a specific quantified potential, whereas other structurally similar diuretics (see Table S-1 in the ESI), such as furosemide (FUR) and hydrochlorothiazide (HCT), presented no oxidative activity. In particular, we explored the relationship between conductance () and electrochemical capacitance ( ) to elucidate the selective responsiveness of TRT to SD-TNT. In addition, the use of impedance-derived spectroscopic methods based


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on the use of immittance spectroscopic functions () allowed the determination of the most appropriate frequency and immittance parameter, and maximisation of the sensitivity to TRT. EXPERIMENTAL SECTION Chemicals, reagents, and equipment All chemicals used in this work were of analytical grade and the solutions were prepared using ultrapure water (18.2 MΩ, Milli-Q® system, Millipore). TRT (purity ≥ 99.9%), furosemide, hydrochlorothiazide, acetonitrile, and dimethylsulfoxide were obtained from Sigma-Aldrich. Potassium dihydrogen phosphate was obtained from Neon, Brazil. Boric, phosphate, and acetic acids were purchased from Merck. Sodium hydroxide and ethanol were obtained from Synth, Brazil. Standard solutions of 0.01 mol L−1 of TRT, furosemide, and hydrochlorothiazide were prepared by dissolution in dimethylsulfoxide, ethanol, and acetonitrile, respectively, and were then diluted in a 0.10 mol L−1 Britton–Robinson (B−R) buﬀer soluXon as the supporting electrolyte. The B−R buﬀer soluXon was prepared by mixing orthophosphoric, aceXc, and boric acid (0.10 mol L−1 each) solutions. The pH values of the buffer solutions were adjusted by the appropriate addition of a sodium hydroxide solution. CV and electrochemical impedance spectroscopy (EIS) measurements were conducted in an Autolab PGSTAT302N potentiostat, equipped with an FRA32 AC module, using NOVA 1.11.2 software (Metrohm AUTOLABx B.V.). Preparation and characterisation of self-doped TiO2 nanotube electrodes The TNTs were prepared by electrochemical anodisation, using an organic electrolyte comprising 0.25% ammonium fluoride (98%, Synth) in glycerol (99.5%, Synth) containing 10% water, conforming with the methodologies published in the literature.20 A Ti sheet with 2 cm × 1 cm area and 0.5 mm thickness was polished using silicon carbide sandpapers of successively finer roughness, degreased by successive sonication for 15 min in isopropanol, acetone, and ultrapure water, and dried in a N2 stream. The Ti sample was then electrochemically anodised under 30 V for 50 h. The TNT electrode was cleaned with deionised water and annealed in air at 450 °C for 60 min. Functionalised electrodes made of SD-TNT were prepared by cathodic polarization using a threeelectrode cell with an Autolab PGSTAT302N potentiostat, Ag/AgCl (KCl 3 mol L−1) as a reference electrode, and a dimensionally stable anode (DSA®, De Nora) as a counter electrode. The electrolyte was a 0.1 mol L−1 KH2PO4 buffer (pH 10) and the TNT electrode was subjected to −2.5 V for 5 min. The area of all electrodes was delimited to 0.28 cm2.26 Subsequently, both TNT and SD-TNT were characterised by X-ray diffraction and field emission gun-scanning electron microscopy. Electrochemical measurements All electrochemical measurements (CV and EIS) were recorded in a 0.10 mol L−1 B–R buffer solution (pH 2) using a conventional electrochemical cell with three electrodes. TNT and SD-TNT-functionalised titanium substrates were applied as working electrodes, and Ag/AgCl (3 mol L−1) and platinum wire were used as the reference and auxiliary electrodes, respectively. The CVs were recorded in the potential range 1.0 to 2.2 V at a scan rate of 100 mV s−1. EIS measurements were recorded from 106 to 0.03 Hz, with a 10 mV RMS (14.14 mV peak amplitude potential or 28.28 mV peak to peak) sinusoidal modulation at a steady-state potential of +1.81 V. The measurements were always performed in triplicate.


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Calculation of conductance and capacitance Spectroscopic methods such as EIS allow us to obtain the time constants of processes and interpret them according to a suitable physical chemistry model. Herein, the time scale of interest is associated with the charging of accessible states in the TNT structures. The relevant relationship is = 1⁄( ), where is the electrochemical capacitance and corresponds to the TNT electronic relaxation obtained as the inverse of the relaxation linear frequency. is the sum of the series resistances, such as that of the solution ( ) (obtained from the Nyquist impedimetric plot47) and the intrinsic resistance ( ) associated with the quantum limit of electronic transport within the nanotubes. The parameters , , and were obtained from the Nyquist capacitive diagrams. The value of of each junction (e.g. TNT, SD-TNT, or SD-TNT/TRT) was experimentally measured from the diameter of the semicircle, as detailed in Figure 2. The value of (16.7 ± 0.5 Ω cm2) was measured from the value of impedance at higher frequencies, as obtained for the titanium metallic substrate in direct contact with the B–R buffer solution (pH ∼ 2.0) prior to the formation of the TNT. was finally obtained by subtracting ( = − ) from which the conductance was estimated as = 1⁄ . Note that the conductance (unit of Siemens, S) and capacitance (unit of Faraday, F) were normalized by the electro-active area of the electrodes.

Conductive and capacitive behaviour of SD-TNT electrodes The conductance and capacitance of the SD-TNT electrodes were interpreted based on the Thomas– Fermi screening analysis of the TNT-modified interface. Debye is only a classical approximation of Thomas– Fermi screening56. In situations where there is a significant accessible nanoscale electronic density of states, such as in SD-TNT, is dominated by (a quantized capacitance), and the electrical field screening is governed by the wave-like characteristics of the electrons associated with the accessible density of electronic states, following the Thomas–Fermi wavevector as the characteristic length of the process governing the storage of charge.56 Accordingly, the associated time scale, for a single electron charging process, corresponds to the time-scale of the electrochemical screening conforming to

= ∑

"

+ ! = ∑

(1)

wherein is, in the present case, the frequency of the electronic relaxation of the accessible electrons (electronic resonance frequency) in the nanotubes which are electronically coupled and driven (or charged) by an applied electrochemical potential in the metallic counter part of the electrode. Note that ∝ ∑ 2% /ℎ = 1/ is the relaxation resistance associated with a quantized conductance () due to the sum of single electron transfer events. Eqn. (1) can be rewritten in the form = / . Eqn. (1)53,56,57 connects the rate of electron transport ( ) and quantized conductance () concepts through the electrochemical capacitance, where is directly proportional to the electronic density of states = % (, where ( is the accessible density of states. As ( changes dramatically for doped and undoped TNTs, the impact of ( on the conductive and capacitive behaviour was evaluated in the modified TNT electrodes (undoped and selfdoped). Impedance-derived spectroscopic analytical methods Impedance-derived methods based on the use of immittance spectroscopic functions () have proved useful as analytical tools.1,2,12,13 By using the approach, the intrinsic electrochemical properties ACS Paragon Plus Environment

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of an electro-active interface can easily be exploited for analytical purposes.47 The parameters are suitable for monitoring changes in the electrochemical response of interfaces, allowing the tracking of the most sensitive (without concerns for the physical meaning of the terms) frequency in an electrochemical impedance spectrum. The concepts have been demonstrated in the tracking of antigen/antibody2,13,48 and lectin/carbohydrate12 interactions. are complex functions, containing real (′) and imaginary (′′) components. The derived functions, such as the modulus (||), inverted function (1/), ratio (′⁄′′), and inverted ratio (′′⁄′), provide a library of twenty-eight constituents to be used analytically. For electro-analytical purposes, the constituents can be normalised by using the -

-

-

-

-

relative response percentage (RR%), which is calculated as , (%) = [( , − 0 )/ 0 ] × 100, where 0 -

represents the blank (in the absence of an analyte) and , represents the value of the constituents after interaction with a specific target concentration (4) at the same linear frequency, f. The responsiveness of the twenty-eight constituents can be evaluated at single frequencies. The sensitivity of the constituents is defined as the slope of the analytical curve obtained at each frequency. This enables the selection of the most suitable condition of the assay in terms of the sensitivity and time required for the measurements (see references 1, 2, 12, 13 for more details). Observe that the complex capacitance [ ∗ (6) = 1/768 ∗ (6)], as used by our research group in other works49–52, acts solely as a particular immittance function which assigns a meaning to the energy storage (faradaic and non-faradaic) terms of an interface. For instance, impedance-derived capacitive measurements were recently applied to interpret the meaning of supercapacitance in electrochemically reduced graphene oxide.53 Here, we reference that work to demonstrate the similarities between the huge capacitance of reduced graphene oxide and that in SD-TNT due to the self-doping procedure.

RESULTS AND DISCUSSION The oxidation of TRT by SD-TNT occurs in the potential interval of 1.81 to 1.9 V against Ag/AgCl and reflects the coupling of TRT electronic states to those of SD-TNT created by the self-doping process. This is noted by observing that there is no oxidation in bare gold, metallic titanium, or glass carbon electrodes. Furthermore, no other molecule to date, e.g. p-phenylenediamine (PPD), furosemide (FUR), or hydrochlorothiazide (HCT), presented any electrochemical activity on SD-TNT electrodes in the potential interval of 1.81 to 1.9 V. There are no reports in the literature, to the best of our knowledge, of any electrochemical activity of similar chemical reactants to SD-TNT electrodes in the same potential interval. For example, in order to monitor the TRT specificity of SD-TNT electrodes, we evaluated the response (in equivalent concentration ranges) of HCT and FUR diuretics using both CV and impedance-derived methods. Within experimental errors, no signal response was observed (see Figure 4a and Figures S-1 and S-2 in the ESI), demonstrating that the SD-TNT specificity is high for TRT. Neither HCT nor FUR diuretics could be detected by CV or the impedance methods (see ESI, Figure S-3).


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−1

Figure 1. (a) Cyclic voltammograms in 0.10 mol L B–R buffer solution (pH 2.0) for undoped TNT electrode (black points), for SD-TNT −1 (red line) and SD-TNT in the presence of TRT in the interval between 5 to 100 µmol L on the SD-TNT surface at a scan rate of 100 −1 mV s . b) The linear response for TRT on SD-TNT (data in ‘a’) was obtained using the peak current (9 ) as the analytical signal.

From the Nyquist capacitive diagrams in Figure 2, the resistance of the nanotube electrodes was obtained (as described in the experimental section). The resistance decreases significantly from ∼161 Ω cm2 (TNT) to ∼70 Ω cm2 (SD-TNT). This corresponds to an increase in the conductance from ∼6.2 mS cm−2 (TNT) to ∼14.4 mS cm−2 (SD-TNT). The electrochemical capacitance, , which is directly obtained from the size of the semicircle in the Nyquist capacitive plot (Figure 2), was 1.9 ± 0.1 mF cm−2 for TNT and greatly increased to 19.2 ± 0.1 µF cm−2 for SD-TNT (Figure 2). The conductive and capacitive values experimentally obtained for TNT and SD-TNT are likely associated with Ti4O7 clusters,43,44 as discussed in the introduction. The self-doping process changes the Ti= ratio and increases the density of the Ti4O7 clusters, which concomitantly raises the conductance and capacitance. TiO2 structures containing different concentrations of Ti4O7 clusters were confirmed to be very different in terms of the density of electronic states by noting that ∝ (.43 Indeed, whereas pure TiO2 is a wide-bandgap semiconductor (bandgap of 3.2 eV), TiO2 containing Ti4O7 clusters behaves as a conductor above 150 K, with donor states located just 0.16 eV below the conduction band. The time-dependent experimental behaviour and electronic relaxation (Figure 2) are associated with the 0.16 eV energetic level acting as the donor, whereas extrinsic oxygen vacancies act as electronic acceptor levels.43 Accordingly, the pseudo-capacitive behaviour of is characteristically dependent on the self-doping process; the capacitance increases from microfarads (per unit of electro-active area) in the TNT to millifarads (per unit of electro-active area) in the SD-TNT (100-fold increase), as shown in Figure 2. The SDTNT capacitive phenomenon observed here was similarly reported by others,32,35 further confirming that SDTNT is a supercapacitive material.32,61 The capacitive (normalised per unit area) charging characteristics reported here are similar to phenomena reported for a distinctly different system53, i.e. graphene oxide (capacitances of the order of microfarads) versus reduced graphene oxide (capacitances of the order of millifarads).53


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Figure. 2 (a) Nyquist capacitance diagrams for TNT, SD-TNT, and SD-TNT in the presence of the TRT target. The variation in the real and imaginary capacitance terms as a function of TRT concentrations is visualised in the Nyquist plot. The measurements were recorded in a 0.10 mol L−1 B–R buffer solution (pH 2.0) in the frequency range from of 1 MHz to 30 mHz, with a 10 mV RMS sinusoidal modulation and DC potential of +1.81 V. was measured at lower frequencies and is the approximate diameter of the semicircle observed in the Nyquist capacitive diagrams. b) Bode capacitive plots for the imaginary capacitance (’’) component. Note that the lower-frequency region of the spectrum is significantly changed in the presence of TRT. Note also that (the electronic resonance frequency) is not affected by the presence of TRT, implying that = / is approximately constant. Nonetheless, there is an evident energy loss process in the frequency region between 86 and 250 mHz (lowest-frequency region) associated with the presence of TRT.

The detection of TRT by the SD-TNT electrode (see Figures 1 and 2) can be explained by the interaction of electronic states of SD-TNT with the nitrogen atoms present in the diuretic molecules. At 1.8 V, an energy alignment is possible within states between the highest occupied molecular orbital and lowest unoccupied molecular orbital levels of TRT and those states in the SD-TNT doping states. The semi-filled electronic state of SD-TNT is an electron conductor along the nanotube length and may allow electrons to flow from the analyte contained in the solution phase to the surface energetic states contained in the SD-TNT solid state, and ultimately to the metallic titanium. It is probable that the alignment of energetic states allows the electrons to flow from the solution phase to energetic solid states, observed electrochemically as the oxidation of TRT (electron holes and acceptor states contained in SD-TNT contribute and serve as mediate states). This oxidation is particularly observed with an increase in the anodic current peak in the CV electrochemical method. Impedance-derived capacitive measurements performed at 1.8 V in the presence and absence of TRT were demonstrative. As shown in Figure 2, with the addition of TRT, the electronic resonance (indicated as ) remains stationary at ∼6 Hz, demonstrating that the electron transfer rate associated with the oxidation of TRT by SD-TNT is unaffected. The physical interpretation is that the amount of the energy loss is equivalent to the amount of energy stored during the electron transfer performed at the solution/solid interface; that is, the ratio between conductance and capacitance, / , is kept constant. Nonetheless, at frequencies lower than those associated with the semicircle of the Nyquist capacitive diagram, i.e. at frequencies between 0.08 and 0.25 Hz (for the lowest frequencies), changes are observed and associated with an increase in the energy loss due to electron charge transport along the nanotubes. The presence of the TRT (at the initial concentration of 0.5 µmol L−1) changed and significantly; decreasing from 14.4 ± 0.02 mS cm−2 and 1.92 ± 0.1 mF cm−2 to 13.1 ± 0.02 mS cm−2 and 1.8 ± 0.1 mF cm−2,


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respectively, in the presence of 0.5 µmol L−1 of TRT, see Figure 2; and proportionally, i.e. the ratio (/ ) of the conductance and capacitance was constant with an average value of 7.3 ± 0.03 (average and standard deviation calculated from three different measurements). These results are shown in Figure S-6 and calculated in accordance with the section calculation of conductance and capacitance as introduced in the experimental section. Subsequent additions of TRT had no noticeable effects on , , or / . In other

words, by successively increasing the concentrations of TRT, up to a limit of 100 µM, the values of and were constant within experimental errors. This confirms that the and parameters are intrinsically associated with the bulk electronic properties of SD-TNT, and that TRT only affects the interfacial states. Accordingly, our interpretation is that TRT adsorbed on the surface (at initial concentrations) of the SD-TNT changes the bulk density of the electronic states, and is measurable by the variations observed in at the initial concentration of TRT. At 1.8 V with the addition of TRT, an electronic conductive path is established (oxidative potential), which allows electron transport from the electrolyte to the solid-state phase. This is supported by the energy loss observed at the lowest frequencies in the Nyquist capacitive diagram (Figure 3a). Furthermore, the quantities , , and / are reversibly associated with TRT quantities, i.e. measurements of , , and / carried out using a similar SD-TNT electrode, but in the absence of TRT (the blank measurement), allowing the full recovery of the CV or impedance-derived capacitance shape. This reversible electrochemical behaviour which depends on the presence of TRT indicates that the bulk electronic structure of SD-TNT is not altered by the oxidation of TRT at the surface. The latter serves only as an electron path for the oxidative process, provided there is alignment between the TRT molecule and SDTNT solid electronic states, rendering the oxidative process exclusively selective to TRT. In summary, based on the conductive and capacitive reversible behaviour of the SD-TNT states as a function of TRT concentration, it may be inferred that the accessible SD-TNT states act as charge carrier paths, allowing acceptor states contained in SD-TNT to overlap with the TRT donor states, thereby favouring an oxidative electronic path. A subsequent increase in TRT concentration only affects the lowest frequencies of the impedance spectrum, which is ascribed to changes in the resistance to electron transport within the SD-TNT solid phase. The changes in this electron transport resistance as a function of the higher concentration of TRT in the electrolyte are not the focus of the present work, and will be discussed elsewhere. In the next section, we demonstrate the optimised quantification of the TRT molecule using impedance-derived spectroscopic methods. Electro-analysis and quantification of triamterene Figure 1b shows the analytical curve of TRT using the peak current (@ ), obtained from CV measurements (Figure 1a), as the transducer signal. A good linear response between 5 and 100 µmol L−1 was obtained, as shown in Figure 1b (r2 ∼ 0.993). We also evaluated the responsiveness of SD-TNT to TRT using the methodology (as detailed in the experimental section) at a potential of 1.81 V. The mathematical analysis was performed using a newly developed MATLAB (The MathWorks, Inc.) program which determined the selection of the most responsive parameters based on the slope of the obtained linear response to TRT. 8’, 8′′/8’, AA , and AA /′ were selected as the most sensitive functions (see Figures S-4 and S-5). Table 1 shows the figures-of-merit of all four functions. The linear relationship was found in the interval between 0.5 and 100 µmol L−1. All functions were one order of magnitude more sensitive than @ . The AA /′ function presents the lowest limit of detection (LoD) combined with a high sensitivity. AA /′, ′′, 8′′/8′, and 8′ showed similar sensitivities of 2.4, 2.3, 1.9, and 2.3 × 106 % decade−1, respectively, whereas


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@ was one order of magnitude lower, i.e. 5.8 × 105 % decade−1. Graphical comparisons are shown in Figure 4b. The LoD values were calculated by considering the standard deviation (SD) of the blank response (over 10 measurements) and slope of the analytical curve, as recommended by IUPAC [(3.3 × SD) / S],62,63 and values of approximately 0.2, 2.5, 0.4, and 2.7 µmol L−1 were obtained for ′′/′, ′′, 8′′/8′, and 8′, respectively. All LoD values were better than that obtained using @ ; 4.1 µmol L−1 (20-fold higher than that obtained for ”/’, for example). Table 1 summarises the figures-of-merit obtained using both the and CV methods. In summary, the results indicated that is analytically better than the CV method (Figure 4b). Accordingly, the use of SD-TNT as an electro-active material and modified electrode for the detection of TRT is promising26, especially when combined with the method. The LoD obtained using ′′/′ satisfies the limit required by WADA.64 The LoD for TRT as reported by others using fluorescence (0.04 µmol L−1),65 high-performance liquid chromatography (0.09 µmol L-1),66 or cyclic voltammetry (0.15 µmol L-1) 46 are smaller than those obtained here using the approach. Furthermore, the combination of impedancederived methods and SD-TNT, as a semiconductive electro-active junction, shows that a sensorial platform can be proposed based on the rational design of electronic states. Semiconductive doped interfaces allow the sensing of small molecules by tracking changes related to both the capacitive and conductive characteristics of electro-active materials. The higher sensitivity attained by the ′′/′ function is not unexpected. This function physically represents the ratio between the energy loss and storage and is thus intrinsically associated with the / ratio. As this is out of the scope of the present work, it will be developed in more detail elsewhere.

Figure 3. Bode plots in terms of the relative response in percentage (RR%) of the DAA/D′ immittance spectroscopy parameter. DAA/D′ presented the best analytical performance during the detection of TRT. b) The magnified region of the diagram; (a) the lower concentration of TRT (black ellipses), where a significant variation between 0.5 to 10 µmol L-1 was observed at lower frequencies −1 (e.g. ∼ 12% to 42% at 0.03Hz). The assays were carried out with target concentrations ranging from 0.5 to 100 µmol L using SD-TNT as an electro-active interface. The measurements were performed over scan frequencies ranging from 0.03 Hz to 1 MHz (x-axes) as a function of the relative response, RR% (y-axes).


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Table 1. Comparative response of the impedance-derived spectroscopic parameters and 9 for the detection of TRT using SD-TNT as an electro-active electrode material.

LoD

EF9G

Linear Range

(µmol L−1)

(Hz)

(µmol L−1)

6

0.2

0.03

0.5–100

6

2.5

0.03

0.5–100

−1.9 × 10

0.4

0.03

0.5−100

6

2.8

0.03

0.5–100

5.8 × 105

4.1

-

5–100

R

C''/C'

0.997

2.4 × 10

C''

0.997

2.3 × 10

Z''/Z’

0.996

Z’

0.996

2.3 × 10

9

0.993

2

S

6

Figure 4 a) Analytical curves acquired at 0.03 Hz using the C”/C’ immittance function as an analytical signal for monitoring the TRT, −1 2 FUR, and HCT responses of the SD-TNT electrode. The responsiveness to TRT concentrations ranging from 5.00 to 100 µmol L (r ∼ 0.997) were very satisfactory, whereas SD-TNT was unresponsive to FUR and HCT for the same range of concentrations. The error bars indicate the variance across the measurements made with three different electrodes. b) Comparison between peak current (Ip) and C”/C’ responses for the TRT target. C”/C’ was the most effective with a 20-fold lower limit of detection (see Table 1).

In summary, the potential of using a rational self-doping TNT semiconductor as a nanostructured material for electrochemical sensing purposes was demonstrated herein. Furthermore, the use of impedance-derived capacitance measurements53–55,58,67 led to improvements in the electro-analytical applicability of TNTs, and further led to useful insights regarding the mechanism accompanying the selfdoping of TNT, especially in the presence of TRT. It is remarkable that the self-doping is elucidated by the

= / relationship, which also explained the concomitant increases in capacitance and conductance associated with the self-doping process of TNTs.


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CONCLUSIONS We demonstrated the applicability of an SD-TNT electro-active semiconductor to the detection of triamterene small molecules using impedance-derived spectroscopy and CV methodologies. The results indicated a superior electro-analytical performance when impedance-based methods are used. Furthermore, it was demonstrated that triamterene is oxidised in contact with the electrode surface of SD-TNT at a specific potential. This enables consistent specificity for triamterene detection compared to other similar diuretics, such as furosemide and hydrochlorothiazide (no oxidation signal was observed for these diuretics in the presence of SD-TNT). Finally, impedance-derived capacitive spectroscopy was used to reveal the origin of the concomitant increase in the conductive and capacitive terms associated with the self-doping of a TNT semiconductor. Both the conductance and capacitance (in semiconductive materials) are related to the electronic time-scale of the electron charging/transport process in the presence of an electrolyte environment. In other words, the sensing properties of SD-TNT to TRT were described using a quantum mechanical first-principle analysis which defines the origin of the electrochemical capacitance ( ). The relationship between and electron transport () is given by the electron transfer rate or electronic resonance ( ). The analysis presented here is evidently extensible to mesoscopic structures (beyond TNTs) that may be coupled to macroscopic electrodes in the presence of an electrolyte. ACKNOWLEDGEMENTS The authors are grateful to the Brazilian Research Agencies ‒ FAPESP (grant numbers 2014/03679-7, 2015/18109-4, 2015/13359-2 and 2012/22820-7) and CNPq (grant numbers 153169/2014-1 and 446245/2014-3) for the financial support granted during the course of this research. PRB acknowledges the Royal Society. FEG-SEM facilities were provided by LMA-IQ and X-Ray Diffraction measurements by GFQMIQ. We additionally acknowledge Amol Patil, for developing the MATLAB R2014a algorithm used in this work. Supporting Information Cyclic voltammograms, Bode and Nyquist plots in the presence of the interferents Hydrochlorothiazide and Furosemide; Impedance-derived analytical curves for Hydrochlorothiazide and Furosemide; Nyquist capacitive and Bode plot for real and imaginary components obtained in the presence of TRT. REFERENCES (1) Patil, A. V.; Bedatty Fernandes, F. C.; Bueno, P. R.; Davis, J. J. Anal. Chem. 2015, 87, 944-950. (2) Bedatty Fernandes, F. C.; Patil, A. V.; Bueno, P. R.; Davis, J. J. Anal. Chem. 2015, 87, 12137–12144. (3) Wu, L.; Gao, Y.; Xu, J.; Lu, L.; Nie, T. Electroanalysis 2014, 26, 2207–2215. (4) Sheng, Z.-H.; Zheng, X.-Q.; Xu, J.-Y.; Bao, W.-J.; Wang, F.-B.; Xia, X.-H. Biosens. Bioelectron. 2012, 34, 125–131. (5) Guo, S.; Wen, D.; Zhai, Y.; Dong, S.; Wang, E. ACS Nano 2010, 4, 3959–3968. (6) Crevillén, A. G.; Ávila, M.; Pumera, M.; González, M. C.; Escarpa, A. Anal. Chem. 2007, 79, 7408–7415. (7) Wang, J.; Li, M.; Shi, Z.; Li, N.; Gu, Z. Anal. Chem. 2002, 74, 1993–1997. (8) Zheng, Y.; Hu, T.; Chen, C.; Yang, F.; Yang, X.; Chen, G.; Hsu, S. M.; Zhang, S. P.; Lasseter, T. L.; Russell, J. N.; Smith, L. M.; Hamers, R. J. Chem. Commun. 2015, 51, 5645–5648. (9) Elshafey, R.; Tlili, C.; Abulrob, A.; Tavares, A. C.; Zourob, M. Biosens. Bioelectron. 2013, 39, 220–225. (10) Daniels, J. S.; Pourmand, N. Electroanalysis 2007, 19, 1239–1257. (11) Lisdat, F.; Schäfer, D. Anal. Bioanal. Chem. 2008, 391, 1555. (12) Santos, A.; Bueno, P. R. Biosens. Bioelectron. 2016, 83, 368–378. (13) Fernandes, F. C. B.; Bueno, P. R. Sensors Actuators B 2017, 253, 1106–1112.


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for TOC only


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Reagentless Detection of Low-Molecular-Weight Triamterene Using

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