Point-of-Care, Cable-Type Electrochemical Zn2+ Sensor with

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Point-of-care, cable-type electrochemical Zn sensor with ultrahigh sensitivity and wide detection range for soil and sweat analysis Sudeshna Mondal, and Chandramouli Subramaniam ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b02173 • Publication Date (Web): 06 Aug 2019 Downloaded from pubs.acs.org on August 6, 2019

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ACS Sustainable Chemistry & Engineering

Point-of-care, cable-type electrochemical Zn2+ sensor with ultra-high sensitivity and wide detection range for soil and sweat analysis Sudeshna Mondala and Chandramouli Subramaniama* a

Department of Chemistry, Indian Institute of Technology Bombay, Powai 400076,

Maharashtra, India Email: [email protected] Keywords: Carbon nanotubes, Receptor membrane, electrochemical sensor, Zn2+ sensor, wearable sensor, sweat analysis, soil analysis.

Abstract

Achieving ultra-high sensitivity over a wide concentration range of analyte has been a persistent, mutually-exclusive challenge for the real-time analytical detection and diagnostics. To this end, we demonstrate a miniaturized, coaxial, cable-type electrochemical sensor comprising of a carbon nanotube immobilized cellulose yarn (CNT-thread) to achieve ultrahigh sensitivity (< 1 ppm) across a wide dynamic range (0.1 – 500 ppm) for the rapid (~ 60 s) detection of Zn2+. The sensor comprises of two cables of CNT-thread - one coated with a polymeric, ion-receptor (tetrakis (p- aminophenyl) porphyrin) acting as the working electrode,

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the other being a reference electrode of pristine CNT-thread. The sensor is extremely tolerant to interference (selectivity coefficient 10-3– 10-5) from a wide range of cations (Na+, K+, Mg2+, Cd2+, Ca2+, Fe2+, Cu2+ ) and anions (Cl-, NO3-, PO43- and CH3COO-) enabling real-time detection of Zn2+ across varied spectrum of samples ranging from human perspiration to agricultural soil. Importantly, excellent signal consistency (< 5% deviation) across multiple electrochemical techniques such as cyclic voltammetry, differential pulse voltammetry and chronoamperometry is demonstrated. Minimal deviation (< 6%) between the analyte concentrations estimated from the sensor and those estimated from atomic-absorption techniques is observed. Further, such ultra-sensitive detection of Zn2+ over a 5000-fold concentration range, is critical for both medical diagnostics (muscular stress) and soil-nutrient assessment (scientific farming). Such a single platform that is capable of handling such diverse real-time applications has not been demonstrated earlier. Finally, the lifetime and mechanical sturdiness of the sensing platform is illustrated through elaborate experiments involving bending and seamless interfacing with human skin for non-invasive point- of-care analysis.


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Introduction The exponential growth of internet-of-things and advanced machine-learning techniques has created a paradigm shift, demanding newer and efficient sensors for the real-time analytical monitoring in widely-varying environments.1 Sectors ranging from personal healthcare2 to agricultural and environmental monitoring have been the major beneficiaries of these advancements,3 demanding versatile sensors and real-time monitoring systems. In this direction, electrical and electrochemical sensors have been the most preferred diagnostic platforms, primarily due to their seamless interfacing between the sensing platform and its signal processing unit. Additionally, their ease of miniaturization and subsequent adaption in varied ambients are distinct advantages over other detection techniques.4 In this context, although optical and fluorescence based sensors exhibit ultra-high sensitivity,5 their poor specificity in practical, real-time situations leaves ample scope for improvement. Accordingly, diverse electrochemical sensors have been developed in the recent years for applications ranging from environmental (heavy metal,6,7 toxic gases8) to healthcare sensors (glucose,9 body temperature,1 body motion10). Non-invasive analysis from various biofluids like tears,11 perspiration12 and saliva13 are gaining increasing attention for such point-of-care applications. In spite of such diversity of applications, the dynamic operational range of electrochemical sensing platforms is often restricted, along with limited sensitivity and specificity. This adversely affects their performance in applications demanding both qualitative and quantitative precision. Although biomolecular receptors in such electrochemical platforms have often improved the specificity and sensitivity14, their shelf-life, reliability and reproducibility are severely affected. Further, uniform immobilization of bio-receptors on the transducing channels is a major challenge and reflects poorly on the signal reliability and quantification. Controlling the molecular density and


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achieving uniform surface coverage of receptors on the transducing surface is a persistent and prominent problem for sensors. Further the same analyte is likely to be monitored across varying concentration ranges depending on the target application. For instance, a multi-faceted mineral such as zinc is both critical for normal functioning of human body and agricultural life although the requirement is sub-ppm level for the former15 and in micronutrient levels in the latter.16 Zinc is the second most abundant transition metal in the human body (2-3 g) and is a primary biomarker for diagnosing the early stage onset of muscular stress and fatigue in humans..17,18 However, non-invasive point-of-care diagnosis of Zndeficiency has proved extremely challenging since its predominant analytical sampling is through perspiration involving microliter sampling volumes at sub-ppm concentrations (0.39 ppm-1.56 ppm).15,17,18 Thus, early stage diagnosis of muscular fatigue requires both accurate and precise monitoring of Zn2+ levels, with extremely small (µL) sampling volume. On the other hand, the Zn2+ requirement for the proper growth and reproduction of crops is 100-fold higher (20 ppm200 ppm).16 While sampling volume is not a bottle-neck for such applications, the specificity of detection is important, since soil samples contain a large variety of chemically similar interfering analytes. Importantly, both these societally and economically critical applications demand ultra-sensitive, rapid and point-of-care detection platform. Thus, developing a universal sensing platform for Zn2+ demands the synergistic combination of high sensitivity, specificity, wide dynamic range and high signal reliability for affordable, qualitative and quantitative realtime detection from low-sampling volumes catering to the internet-of-things. Addressing this demand, we report a point-of-care, portable electrochemical platform for the non- invasive, qualitative and quantitative monitoring of Zn2+ from chemically varied spectrum of analytes ranging from human perspiration to soil. Catering to this wide spectrum, the electrochemical platform demonstrated here exhibits reliable signal (relative standard


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deviation~2.5%) across a concentration range spanning three orders of magnitude (0.1 ppm-500 ppm), wide pH range (4.5-7.0) and widely varying chemical analytes (human perspiration to soil). The electrochemical sensor consists of free-standing scaffold of cylindrical cellulose 4 fibers containing a uniform coating of single-walled carbon nanotubes (CNT-thread, diameter 150 µm, length - 10 cm, and linear density 0.125 mg.cm-1). Such CNT-thread exhibits synergistic combination of high specific surface area (800 m2.g-1), uniform electrical conductivity (25 S.cm-1) and ideal polarizibilty over a wide voltage window (-1 to 1 V). This ensures its usability as both the working electrode and the reference electrode, in addition to being mechanical flexible and robust. Ultra-high sensitivity (108 molecules of Zn2+) and signal reliability (RSD~2.5%) is achieved by a uniform co-axial coating (thickness - 1.6 µm) of receptor containing polymeric matrix over the CNT-thread. Besides ensuring a synergism with the transducing CNT thread, the polymeric coating achieves uniform receptor density (7.86 x 1011 molecules.cm-3, details provided in Scheme S1) over the sensing surface and thereby yields reliable performance.19,20 The polarizable electrochemical nature of the CNT-thread and the polymeric matrix ensures its stable performance as both the working and the reference electrode. Such a sensor is amenable for operation through a wide range of electrochemical detection techniques ranging from steady-state techniques such as cyclic voltammetry to transient techniques such as differential pulse voltammetry and chronoamperometry. Quantitative and qualitative detection of Zn2+ is achieved through diverse electrochemical techniques, establishing its effectiveness as a generic and universal detection platform. Extremely low value of selectivity coefficient (~10-3- 10-5) is achieved for a range of potential interferents (Fe2+, Cd2+, Mg2+, K+, NO3-, and CH3COO-) implying high selectivity and comprehensive adaptability for real-time applications. Accordingly, rapid signal response (< 1


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min) towards real-time analysis of (a) human sweat samples during various stages of physical activity and (b) soil samples from various geographical locations of India (deep black soil from Maharashtra, red loamy soil from Rajasthan, red clayey soil from Tamil Nadu) is demonstrated using the CNT- thread sensor. This sensor operates over a wide dynamic range (0.1 ppm-500 ppm) and thus can be effectively multiplexed. Thereby, the CNT-thread is established as an efficient point-of-care device for personal healthcare and on-field agricultural monitoring, hence proving its effectiveness in providing insightful information about its utilization in both healthcare and environmental applications.

Figure 1. Schematic representation of the fabrication process of the sensing device starting from (a) single walled carbon nanotube (CNT) forest, (b) dip-coating the cellulose thread into the CNT ink to form the counter/reference electrode, (c) further dip-coating into ionophore (TAPP) solution forming the working electrode and (d) final integration into laminated platform. The device is fabricated as a rectangular platform for perspiration analysis while it is integrated as a circular disk for the soil sensor.

Experimental Section


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Super-growth single-walled carbon nanotubes (CNTs, length : 500 µm, diameter : 3 nm) were synthesized using a water-assisted chemical vapour deposition technique.21 Tetrakis (paminophenyl) porphyrin (TAPP), high molecular weight poly(vinyl chloride) (PVC), tetrahydrofuran, 2-nitrophenyl octyl ether (NPOE, >99%), Sodium tetraphenyl borate (NaTBP, > 99%), and analytical grade of the following salts Zn(NO3)2, Ca(NO3)2, Fe(NO3)2, Mg(NO3)2 and sodium deoxycholate (NaDOC) were purchased from Sigma Aldrich and used without further purification. Acetate and phosphate buffers were used to regulate the pH of the medium. Ultrapure Milli-Q water was used throughout the study. Preparation of tranducer: CNT-thread Aqueous conductive inks of CNTs were prepared by dispersing them through probe sonication (50% power at 26˚C using PKS-750FM, PCI Analytics) using NaDOC as an anionic surfactant. The similarity in solubility parameter between supergrowth CNTs and NaDOC ensured uniform dispersion with minimal agglomeration.22 The average length and diameter of the tubes were 500 µm and 3 nm respectively, with minimal change in structure of the CNTs as indicated by invariant ID/IG ratios.22 Such uniform, conductive dispersions were used as inks through which commercially available cellulose threads were dip-coated. This results in uniform immobilization of CNTs thereby creating a continuous, electrically conductive pathway. This was subsequently dried in air, washed in ethanol and redried in air to form CNT-thread which acts as a transducer for all the sensors used in this study. Preparation of receptor: ionophore membrane The CNT-thread was dip-coated in a ionophore cocktail prepared using PVC, ionophore TAPP, plasticizer NPOE and anion-excluder NaTPB (30:5:55:10) dissolved in 5 ml of tetrahydrofuran. The CNT thread was then air dried, resulting in the formation of a uniform coating of ionophore


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membrane onto it. This acted as a chemoreceptor for analyte detection. Thus the receptor was in direct contact with the transducing CNT thread. Thereby any chemical changes to the receptor film are efficiently transduced by the CNT-thread. Material characterization The morphology of the CNT-thread and TAPP membrane coated CNT-thread was studied using field-emission secondary electron microscopy (FE-SEM, Zeiss ultra 55 FESEM, 5 kV). X-ray photoelectron spectroscopy was recorded in AXIS Supra (Al Kα 1486.6 eV). Confocal Raman spectroscopy was recorded with WiTeC MicroRaman using 532 nm Nd-YAG excitation laser source (5 mW power). The signal was collected in a confocal, back-scattering geometry, passed through a super-notch filter and subsequently dispersed using an 1800 grooves/mm grating onto a Peltier cooled CCD. Confocal spectral mapping of the samples was carried out at diffractionlimited spatial resolution by mounting the samples on piezo controlled XYZ translational stage. Each spectral image corresponds to 10,000 spectra. The spectral maps are over an area of 625 µm2 is converted to binary scale using ImageJ to assess the quantitative spread of the analyte over the receptor membrane. Device fabrication The CNT-thread/TAPP membrane sensor platform was built on a laminated, polyethylene terephthalate (PET) flexible platform. The TAPP-membrane coated thread and CNT-thread acts as the working electrode and the reference-counter electrode, respectively. These were attached to conductive copper tape and sandwiched between two laminated PET sheets to fabricate a simple, cost- effective device, with dimension comparable to that of a two-rupee coin (Figure 1). A circular opening (4 mm diameter) acted as the analyte-collection/sampling area. Electrochemical Measurements


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All electrochemical measurements were conducted using Biologic SP-300 Potentiostat in a twoelectrode configuration with the ionophore-membrane coated CNT-thread acting as the working electrode and the pristine CNT-thread acting as reference electrode. Control experiments with CNT-threads acting as both working and counter electrodes were conducted to estimate the operational electrochemical voltage window. All analyses were performed after removal of oxygen by bubbling N2 for 5 minutes and the results collected after iRdrop corrections. Real-time sampling and analysis The real-time analysis of perspiration samples were conducted by collecting samples from a healthy volunteer while performing various forms of exercise like running, skipping and aerobics. An absorbent cloth was tied to the wrist of the donor during the period of physical activity to collect the perspiration. 50 µL of perspiration was collected at four different instances spread three times across the duration of exercise. Subsequent point-of-care experiments were conducted by directly attaching the sensor strip on the skin. Soil samples were collected from various parts of India and categorized into red loamy soil, deep black soil, and red shallow clayey soil. Two grams of each soil sample was dispersed in distilled water and filtered to prepare the samples for the real-time estimation of Zn2+. Results and Discussion The sensitivity and reliability of any analytical platform is strongly dependent on the receptivity of the transducer and the uniformity of the receptor-analyte interface. Chemically inert, electrically conductive nanocarbon based materials have therefore been prominent choices as transducers.23 However, self-assembly mediated ensembles of such nanocarbons to create transducing surfaces have often lead to their uncontrolled aggregation (π-π stacking and van der Waals) resulting in severe deterioration of the surface area and thereby adversely affecting its


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sensitivity and reliability. Here, we overcome this obstacle by designing a cellulosic fiber based platform for capturing and immobilizing CNTs from CNT-ink (Figure 1). The scalable, dip-coating route enables the formation of uniform and interconnected, interpenetrating network of CNTs covering the cellulosic scaffold. Such an immobilization converts a perfectly insulating cellulose scaffold (10-18 S.cm-1) to an electrically conducting CNT- thread (25 S.cm-1) resulting in a conductivity enhancement of 1019. The uniformity in coverage and the formation of an interconnected network is confirmed by invariant electrical conductivity estimated across multiple segments of the CNT-thread, varying in length by over three orders of magnitude (1 mm to 100 cm, Figure S1). Significantly the CNTs immobilized on the substrate do not exhibit any extensive aggregation as confirmed through the surface area measurements (BET SSA~900m2.g-1).3,24 The presence of the CNTs spread over a macroscopic area while simultaneously retaining its electrical conductivity and specific surface area is an important attribute that is demonstrated here enabling its use as an effective and sensitive transducing surface.


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The second important challenge while designing reliable electrochemical sensors lies in achieving a uniform and reproducible surface density of receptor molecules. The signal reliability and reproducibility is strongly correlated to the number density of receptor molecules and its uniform-coverage over the transducing surface. This critical requirement is satisfied by employing a polymeric scaffold into which the receptor molecules are uniformly dissolved in the solution phase. Such polymer incorporated receptor systems function as the sensing surface and are seamlessly interfaced with the transducing CNT-thread in the form of a membrane made from PVC (Figure 1). NaTBP along with anion-excluder NPOE is also included in the receptor membrane to achieve uniform cross-linking and minimize the interference from anions (Figure 1). Several complimentary experiments were carried out to establish the uniformity of receptor coverage. Since TAPP exhibits the characteristic Soret bands with well-defined extinction coefficient, the uniformity of its immobilization was assessed by monitoring its absorbance at varying locations (Figure S2). An unchanged absorbance (0.85 at λmax = 450 nm) from such measurements confirms the homogeneity in the distribution of TAPP and uniformity in the thickness of the receptor membrane (Figure S2). Importantly, the TAPP number density is estimated as 7.86 x 1011 molecules cm-3 (Scheme S1). Comparing the solid-state and solutionstate absorption of the TAPP provides important evidence for the nature of interaction between the receptor (TAPP) and the analyte (Zn2+). The pristine TAPP exhibits two distinct Q-bands originating from vibrational excitations. In contrast, the Zn metallated TAPP exhibits a single Q-band attributed to the S0-S1 transition, indicating charge-transfer interactions (Figure S3).


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Raman scattering provides a complimentary evidence of the seamless interface created between the CNT-thread and TAPP receptor membrane (Figure 2(a)).The Raman spectrum of CNT-thread exhibited three characteristic peaks at about 1345 cm-1, 1591 cm-1 and 2662 cm-1 corresponding to the D and G and 2D bands, respectively. In comparison, the TAPP receptor coated CNT-thread exhibited additional peaks at 997 cm-1, 1115 cm-1 and 1500 cm-1 originating from the ν(C=C) and ν(C=N) vibrational modes. The invariant spectral positions of the G- and D- bands confirms the structural stability of the TAPP receptor covered CNT-thread.

Figure 2 (a) Raman spectra of CNT-thread and CNT thread- TAPP receptor membrane. (b) Absorbance spectra of the TAPP receptor membrane before and after Zn addition (ZnTAPP). SEM images of (c) the pristine CNT-thread and (d) CNT thread- TAPP receptor.


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Interestingly, encapsulation by the TAPP membrane increases the content of sp2 C=C leading to enhancement of the G band intensity (IG/ID changes from 1.1 to 1.3). This is due to the combined intensities of the sp2 C=C of the CNTs and the TAPP membrane, sampled in the confocal geometry. Therefore, the increase in the G-band intensity is correlated to the higher number of sp2 C=C scattering centers within the sampling volume and is not due to any additional bond formation.

Figure 3. (a) Raman spectra of TAPP receptor membrane before (black trace) and after (red trace) addition of 25 µL of 0.5 M Zn2+ solution. Spectra are shifted vertically for clarity. Inset shows the Raman spectral map of the TAPP receptor membrane before Zn2+ addition, based on 1598 cm-1 (G band). Raman spectral maps constructed based on G band (blue) and fluorescence of TAPP (3719 cm-1, red) before (b) and after (c) addition of 25 µL of 0.5 M Zn2+ solution.


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The effectiveness of the receptor coating is also established through SEM images reveals the uniform adsorption of the polymer onto the CNT thread (Figure 2(c), (d)). Further, evidence is provided by the uniform electrical conductivity (25 S.cm-1), high specific surface area (800 m2g1

) and porosity (0.7 nm, 3 nm).3 (inset, Figure 2(d)). The ability of the pristine TAPP receptor membrane to selectively capture Zn2+ is investigated

through spectroscopic and microscopic techniques. The broad Soret bands observed for the TAPP- receptor membrane transforms to sharp and distinct Soret peaks upon binding with Zn2+ (Figure 2 (b)). The bathochromic shift in the spectral position and its simultaneous sharpening confirms the charge transfer between the immobilized TAPP and the Zn2+ in the analyte. Such significant changes in the absorption of the receptor membrane confirms the ability of the porphyrin cavity to coordinate Zn2+ in spite of being immobilized in the polymeric matrix.25,26 Such an interaction is driven through the pπ-d orbital interactions and is further facilitated by the porous nature of the receptor membrane (Figure 2 (d)). Raman spectrum of the TAPP receptor membrane exhibits the D- and G- band corresponding to the polymeric backbone, in addition to the distinct fluorescence originating from the porphyrin moieties of TAPP. As expected, the fluorescence intensity is significantly higher compared to the Raman scattering intensity (Figure 3 (a), black trace). Accordingly, the chemical composition of the TAPP receptor membrane is reconstructed using the Raman signatures (inset, Figure 3 (a)) and exhibits identical morphology to the SEM images (Figure 2(d)). Further, overlapping of spectral maps based on (a) Raman modes of the polymer (blue region, Figure 3 (b) and (c)) and (b) fluorescence from the TAPP (red region, Figure 3 (b) and (c)) identified mutually exclusive and distinct domains. The overlap of these spectral maps (Figure 3(b)) clearly identifies the uniform localization of the ionophore within the polymeric


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matrix. Interestingly, the addition of Zn2+ to such a sample resulted in the sharp quenching of the fluorescence with complete retention of the Raman signatures. Specifically, the intensity ratio of fluorescence to Raman decreased from 2.3 to 1.2 due to addition of Zn2+ indicating an effective quenching of the fluorescence and confirming the specific charge-transfer interactions between the analyte and receptor membrane. Concomitant with the decrease in the fluorescence intensity, the spectral map also revealed a significant reduction in the area occupied by the unbound porphyrin (from 53% in Figure 3(b) to 23% in Figure 3(c)). Such a spectral mapping conclusively establishes the metallation of the porphyrin by Zn2+ leading to both quenching of the fluorescence and decrease in the fractional coverage of the unbound porphyrin in the receptor membrane. The nature of charge transfer interaction between Zn2+ and the receptor membrane is further investigated by X-ray photoelectron spectroscopy. Corresponding to changes in absorption characteristics, distinct changes in the binding energies of C and N are observed (Figure 4(ac)). It is to be noted that the C1s XPS of the pristine receptor membrane exhibits a broad peak centered at 286.6 eV that consists of a predominant C1s component (286.6 eV) and a lower binding energy component corresponding to electrophilic carbon (C=N at 285.2 eV).


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Figure 4. (a) XPS spectra for C1s in TAPP-receptor membrane before (TAPP) and after zinc (Zn-TAPP) insertion into the ionophore. (b) N1s XPS spectra for TAPP-receptor membrane before (TAPP) and after zinc (Zn-TAPP) insertion into the ionophore. (c) Zn 2p XPS spectra before (TAPP) and after zinc (Zn-TAPP) insertion into the ionophore. (d) I-V plots for demonstrating conductivity of cellulose thread, CNT-TAPP receptor membrane and CNT-thread. Thus the C1s peak is derived from both the porphyrin and the polymeric moieties of the receptor membrane and therefore accounts for the peak broadening. In contrast, the N1s is specific to the porphyrin receptor and also is the site for the Zn2+ coordination. Therefore, the coordination of Zn2+ is reflected predominantly through changes in the N1s peak. The N1s peak in pristine TAPP receptor membrane is observed at 409 eV and 401 eV corresponding to the N1s of


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pyridinic nitrogen; upon binding of Zn2+ the peak exhibits a shift to lower binding energy indicative of charge transfer from the Zn2+ ion to the porphyrin cavity mediated by the pyridinic nitrogen (Figure 4(b)). This observation is in excellent agreement with earlier reports and binding events occurring in biological processes and establishes the ability of the TAPP-receptor membrane to selectively capture Zn2+ from the analyte solution. The CNT thread as the transducing platform is uniformly coated with the TAPP receptor membrane, enabling it to comprehensively act as the working electrode for electrochemical detection. The performance of this electrode is critically dependent on the interface between the CNT thread (transducer) and the receptor membrane (sensor). The extensive and uniform infiltration of TAPP receptor membrane into the CNT-thread is reflected as decrease in the conductivity of the CNT-thread (Figure 4d). The TAPP receptor membrane acts as a dielectric separator between the individual fibrils of the CNT-thread and there-by causes a lowering in the electrical conductivity. However, the Ohmic behavior of the CNT-thread is retained after the TAPP receptor coating and is critical to enable direct electrochemical measurements without any Schottky defect. In spite of this, the volume conductivity and fiber resistivity of the CNT-thread are significantly higher than several other reports27-29 and thereby represents a distinct advantage to the performance metrics of the sensor. The combination of high microporous surface of the CNT-thread for high sensitivity and uniformly percolated TAPP receptor membrane providing chemical selectivity, conclusively confirms the ability of the entire sensing platform to respond to Zn2+ and its concentration fluctuations. In order to evaluate the performance of the CNT thread-TAPP receptor membrane (working electrode, WE) as an electrochemical sensing platform, several control experiments were performed. As the WE, the primary requirement of the transducer is to remain electrically


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conductive, reversible and ideally polarisable under the conditions of the experiment. Similarly, the TAPP-receptor membrane has to specifically interact with the Zn2+ through Faradaic chargetransfer interactions. Varying the thickness of the receptor membrane, sheathing the CNT thread is observed to produce marked changes on the redox behavior and in particular to the redox current at the WE (Figure S4). The linear decrease in the magnitude of current is observed with increase in thickness of the TAPP-receptor membrane. Thicker TAPP receptor coating increases the separation between the analyte-receptor interaction site and the transducing surface. Therefore, the Zn2+- TAPP charge-transfer interaction occurring at the surface has to diffuse through a larger thickness of the polymer and hence results in decreased current. Alternatively, reducing the thickness of the TAPP- receptor membrane results in non-uniform coverage and consequently fluctuating redox processes leading to unreliability of the sensor. Thus, a trade-off between sensitivity and reliability was achieved by employing an optimal thickness of TAPP receptor membrane (1.6 µm).

This afforded dimensional uniformity,

reliability and reproducibility of signal response.


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Figure 5. Cyclic Voltammograms for the sensor at various Zn2+ concentrations (a) 0.1 ppm0.5 ppm. (b) 10 ppm-50 ppm. (c) 100 ppm-500 ppm. (d) Calibration plots of the cyclic voltammograms at pH 4.5 and pH 7.

The electrochemical performance of the pristine CNT thread is compared with glassy carbon (GC) electrode for a well-established Fe2+/Fe3+ redox system (Figure S5) to evaluate its performance in charge transduction. The primary requirement of the CNT-thread is its ability to transfer charges across the interface onto the electroactive species in the electrolyte without undergoing any electrochemical Faradaic reaction. Further, reversibility of such electrochemical reactions mediated by the transducer needs to be maintained during voltammetric cycling. A peak-to-peak potential difference (ΔEp) of 270 mV and 161 mV is observed in


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case of CNT-thread and GC, respectively. This confirms the ability of the CNT-thread to effectively transduce the charges across the sensor interface. Similarly, the Faradaic redox reaction (Fe2+/Fe3+) occurs at similar values of potential in both the CNT thread and GC. However, the current observed in case of GC is seven times higher than that with CNT-thread, although both these currents scale linearly with the concentration of the analyte. This arises from the highly polished and crystalline surface of GC that has higher intrinsic electrical conductivity (2000 S.cm-1) than the CNT-thread (25 S.cm-1). Further, the CNT-thread exhibits a prominent capacitive current arising from the formation of electrical double layer. In spite of higher transducing current, the use of GC for real-time applications is limited due to its limited availability, cost, mechanical rigidity and brittleness. This assumes greater significance in point-of-care and on-field applications where the CNT-thread exhibits distinct advantages such as mechanical flexibility and chemical inertness. Importantly, the presence of an electrical double-layer at the CNT-thread/electrolyte interface is critical from the perspective of electrochemical sensing, since it signifies greater availability of the electroactive species and consequently higher sensitivity of detection. The system showed reversible diffusion kinetics with increase in current at higher scan rates following Randles-Sevcik equation. Finally, no other Faradaic reactions are observed with the CNT-thread indicating its chemical inertness and ideal polarizability. Having established the capability of the CNT-thread for sensitive electrochemical transduction and the absence of its involvement in any Faradaic process within the electro-chemical window of interest, the sensor consisting of the TAPP receptor membrane coated CNT-thread was used for monitoring Zn2+ concentration in aqueous samples. Cyclic voltammetry experiments are per-


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formed in a two electrode sensor assembly with TAPP receptor membrane coated CNT-thread functioning as the working electrode and the pristine CNT-thread acting as counter-reference electrode. The ability of pristine CNT-thread to behave as an ideally polarised electrode (whose potential does not change with current flow) enables its use as both the counter and reference electrodes. The synergistic combination of sensitivity from the CNT thread and specificity from the ionophore membrane is utilized in the detection of Zn2+ over a wide concentration ranging from 0.1 ppm - 500 ppm. A spectrum of samples ranging from agricultural soil to human perspiration were employed for this study. Accordingly, the performance of the sensor was also evaluated across a wide pH range (pH 4.0-7.0) and wide concentration range spanning three orders of magnitude (0.1 ppm to 500 ppm) that is normally encountered in such applications. Human perspiration was simulated in laboratory condition for these initial measurements using acetate buffer containing known concentrations (ppm level) of Zn2+ corresponding to the acidic pH of human perspiration. Similarly both phosphate and acetate buffer containing standardized concentrations of Zn2+ were used to access the performance of the sensor for agricultural applications. The cyclic voltammograms in all these experiments exhibit distinct redox peaks at 0.4 V and -0.2 V corresponding to the electrochemical redox activity of Zn2+ binding with the porphyrin core of the receptor membrane (Figure 5 (a-c), Figure S6). The charge transfer occurring during this binding event (as observed in absorbance and XPS measurements) is dedicatedly transduced by the CNT thread across the entire concentration and pH range. Importantly, stability and reliability of the sensor is established through the invariant redox currents through multiple runs (thrice) conducted for each Zn2+ concentration and pH of the analyte solution. The signal fluctuation is estimated from the relative standard deviation of the measured peak


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currents (cathodic and anodic) to be < 2.5%, signifying the stability and reliability of the sensing platform (Table S1). Such stable and reliable performance of the device is attributed to three main reasons. Firstly, the uniform and high conductivity of the CNT-thread ensures identical and equipotential transducing surface. Although transducers with higher electrical conductivity have been reported, the uniformity of charge transduction over a length scale of several tens of centimeters along with high SSA (for higher sensitivity) that is demonstrated here has not been achieved earlier. Secondly, the homogeneous coating of the TAPP-receptor membrane achieves a uniform covering of the receptor molecules to form the sensing surface. Further, the high number density of the TAPP receptors (7.86 x 1011 molecules. cm-3) and its extensive interface with the CNT-thread implies highly stable and ultra-sensitive detection. Finally, the standardization of the sampling area using a simple, reproducible and well-defined PET boundary ensures minimal signal variation. Further, the response of the sensor towards the wide concentration range of the analyte (Zn2+) exhibits two distinct linear domains that are invariant with the pH of the analyte. The initial domain corresponds to the concentration of Zn2+ lower than 100 ppm wherein the sensitivity of the sensor corresponds to 60 µA/ppm (Figure 5(d)). At concentrations higher than 100 ppm the sensitivity exhibits a marked increase to 1000 µA/ppm (Figure 5(d)). The signal sensitivity obtained in this report is among the highest reported for different analytes such as Zn2+, glucose and lactate in varying samples (Table S2). The linearity of the device extended to the entire concentration range (albeit with different sensitivity) without exhibiting any saturation. Further, complete recovery of the sensor and its reusability for multiple times is achieved by washing the device in dilute HCl to remove the Zn2+ from the TAPP cavity (Figure S7). The distinct slope of the RandlesSevcik plot also confirms the specificity of the sensor towards Zn2+ (Figure S8 (a-b)).


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The fundamental sensing operates through a confluence of two pathways: (a) the sizeselective capture of Zn2+ by TAPP receptor membrane, and (b) the specific charge-transfer based interactions between Zn2+ and TAPP. It is important for both these conditions to be satisfied in order for signal generation and further transduction. Thus, cations such as Na+, K+, Ca2+, Mg2+ that are present in large excess in the buffer (1000 times higher concentration) and are size-compatible with the porphyrin cavity do not give rise to Faradaic current due to the absence of any charge-transfer interactions with the TAPP- receptor. On the other hand, Zn2+ exhibits both the ideal ionic radius and the optimal positioning of the HOMO-LUMO levels to enable facile pπ-d interactions leading to charge transfer interactions. Such a charge transfer based metallation of the porphyrin is precisely captured by the high surface area of the traducer. Absorption studies carried out with pure TAPP and Zn2+ in solution state confirms the underlying basis of such a mechanism as discussed before. (Figure 2(b)). Thus, the specificity has a chemical origin while the sensitivity originates from the transducer. The effectiveness of the sensor is reflected in its ability to be accessible through a variety of electrochemical measurements without being restricted to cyclic voltammetry (CV). Accordingly, we demonstrate both steady-state and transient methods occurring at the microelectrodic surface. Signals from differential pulse voltammograms (DPV), arising due to an initial pre-concentration/ deposition of Zn2+ followed by its subsequent stripping at a -1.2 V, provides an unambiguous current that is proportional to the bulk concentration of Zn2+.(Figure 6 (a-b)) As opposed to cyclic voltammetry, the interference from capacitive double layer charging is minimized with DPV. As in case with CV, no current saturation was observed, indicating the robustness of the sensor and its ability to handle wide concentration range. Chronoamperometry was employed as a transient technique to evaluate the working of


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sensing platform in real-time. The applied potential was maintained close to the reduction potential of the sensor (-0.4 V) while the transient current was monitored (Figure 6 (c)). The potentiostatic current followed an exponential growth leading to stabilization within a minute, irrespective of the concentration of the Zn2+ employed. The transient fraction of the current (measured at 5s) exhibited a distinct linear dependence on the concentration of Zn2+ (Figure S9). As expected, this difference was reduced in the steady-state part (measured at 50 s) 4 although maintaining its linearity. Importantly the steady-state current was reached within a minute indicating the fast response of the sensor and its applicability in real-time. The linear dependence of the transient current on analyte concentrations indicates its wide dynamic range of sensing and corroborates with earlier CV and DPV measurements. Further, the sensor exhibited high selectivity towards Zn2+ inspite of the presence of a large excess (hundred times more) of several other ions such as Na+, K+, Mg2+, Ca2+, Cl-, Fe2+, Cu2+, Cd2+, NO3-, PO43- and CH3COO-. These ions constitute the most common interferents encountered in real-time analysis of soil nutrition and non-invasive perspiration based medical diagnosis (Figure 6(d)). The selectivity coefficient of the sensor estimated from these measurements ranges from of 2 x 10-5 for NO3- to 0.35 x 10-3 for Fe2+. Such low selectivity coefficients for electrochemical measurements indicate the robust nature of the detection protocol and the extreme reliability of the CNT-thread/TAPP membrane device as the sensor platform.


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Figure 6. (a) Differential pulse voltammograms of Zn2+ on the sensing device (Deposition potential: -1.2 V), (b) Plot of peak currents versus concentration of Zn2+ for DPV measurements. (c) Chronoamperometry measurements on the sensing device. (d) Plot of Zn2+ selectivity among common interferent ions.


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After achieving reliable and stable detection of Zn2+ using the sensor, the final confirmation of its performance is evaluated in real-time, on-field conditions. The wide operational range exhibited by this sensor implies that it can be used for both direct, non-invasive, perspiration based medical diagnosis and rapid, precise assessment of soil nutrition levels. Accordingly, both these wide-ranging applications are demonstrated using the sensor patch. The assessment of Zn2+ levels from perspiration is an established bio-marker for early-stage diagnosis of muscular fatigue.17,18 Given the low sampling volumes, in-situ, non-invasive measurements are mandated for such applications. Therefore the sensor in the form of a stickon patch is attached on the surface of kinesologic tape for conformal attachment to the human skin. The sensor is placed at the bottom of the wrist, as per established conventions for collecting the analyte (perspiration). The presence of the sensor does not restrict the movement of the user in any manner and simultaneously is effective in analysing the perspired sample (Figure 7(a)). Periodic CVs measured over 1 hour with ~ 50 µL of sampling volume exhibit distinct redox peaks corresponding to electro-chemically active Zn2+ (Figure 7(b)). The concentration of Zn2+ in such samples are expected to increase with time of exercise due to build-up of muscular stress. This is dedicatedly reflected in the peak currents of the CVs that increase with increase in exercise time. Importantly, the entire measurement and analysis is carried out on body fluid without any separate sample preparation or pre-concentration. The Zn2+ concentration is estimated from such measurements using the calibration plot established earlier (Figure 5 (d)). Importantly, such calibration plots derived from laboratory conditions offer excellent qualitative and quantitative information from real-time samples. The ultrasensitivity obtained in Figure 7 (b) with real-time samples originates from the


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effectiveness of the extensive TAPP-CNT-thread interface (receptor-transducer). This ensures that any charge-transfer induced Zn-TAPP interaction is transduced by the CNT-thread to achieve ultra-high sensitivity.


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Figure 7. (a) Demonstration of the wearable device for collection of analyte (perspiration). (b) Cyclic voltammetry analysis on real-time perspiration samples from a healthy donor collected during performance of physical activity (aerobics). (c) Photographs of deep black soil (left), red loamy soil (middle) and red clayey soil (right) collected and kept overnight for sample preparation. (d) Chronoamperometric tests conducted with the device using the supernatant solution from the soil samples. (e) Graph showing performance of the device in comparison to concentrations obtained from ICPAES. (f) Plot of Current vs bending angle on the sensor demonstrating the flexible nature of the wearable sensor. This establishes that the presence of other biomolecules and possibly cellular residues does not interfere with the performance of the sensor in any manner. Ultra-sensitive detection of Zn2+ from perspiration over a concentration range of 0.003 ppm to 0.015 ppm is thus achieved. Benchmarking such an analysis with ICP-AES for the same samples establishes a deviation of < 6 % indicating complete operational robustness and reliability of the sensor. Thus, this sensing platform outperforms several similar electrochemical devices in terms of (a) sensitivity,30,31 (b) reliability,32 (c) quantification and qualification,33-34 (d) sampling volume15 and (e) real-time operability.9 This demonstrates the point-of-care capability of the sensor along with its resilience to external conditions. Soil represents a biologically more diverse analyte compared to perspiration, besides posing challenges due to its solid phase. Therefore the analysis of Zn2+ levels in soil is carried out by extracting the nutrients from the soil using water. The estimation thus carried out would relate to the concentration of nutrients that are available to agricultural crops, since the predominant uptake of minerals occurs through their aqueous, dissolved forms. Three different soils namely,


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deep black soil, read loamy soil and red clayey soil collected from geographically different parts of India (Maharashtra, Rajasthan, Tamil Nadu) are mixed with water (Figure 7(c)). The supernatant thus obtained is used for all further testing. In addition to demonstrating the capability of the sensor to operate with diverse samples, the analytical versatility of the device is established through chronoamperometric tests (Figure 7(d)). Herein, the sensor is redesigned in the form of a circular disk with an opening for the introduction of analyte (inset, Figure 7 (d)). As in the case with all previous studies the chronoamperometric response stabilizes within a minute, with each sample giving a distinctly unique stabilisation current. The final and most rigorous performance testing of the device is carried out by mechanically bending the sensor at various angles and recording its response. The signal response, in terms of current, is found to be invariant indicating the reliability of the sensor under mechanical stress (Figure 7 (f)). Importantly, such stable signal output under flexing indicates the robustness and strength of the CNT membrane interface. Any change in the interface would be reflected as fluctuating, irregular signals which is not observed at any point of testing and sampling. Therefore, achieving such stable performance in different mechanical stresses is possible due to the inherent pliability of both the CNT-thread and the polymeric coating (TAPP membrane). Conclusions In summary, we demonstrate a versatile cable-type, electrochemical detection platform with high qualitative accuracy and reliability over a wide concentration range. The ease of use, mechanical sturdiness and high qualitative and quantitative accuracy implies field-deployability for applications ranging from medical diagnostics to scientific farming. High specificity across a plethora of interfering cations and anions, with rapid and reliable signal generation augers well for direct commercialization. Finally, the design and concept of the detection platform achieved


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here is amenable for a variety of other analytes, opening a whole domain of transformative opportunities towards Internet-of-Things. Associated content Supporting Information : Tables comparing the performance of CNT-thread as sensor with other reported and similar sensing platforms, variation in electrical conductivity of CNT-thread with its length, absorption spectra recorded at varying positions of the TAPP-receptor membrane, estimation of TAPP receptor density, absorption spectra of TAPP solution before and after addition of Zn2+, variation in signal-current from sensor at varying thickness of TAPP receptormembrane, comparative performance of CNT-thread and GC, demonstration of reusability of the CNT-thread sensor, working of CNT-thread at varying scan rates, Calibration plot of transient current with varying concentrations of Zn2+, CVs recorded with sensor at pH of 7, Table showing the current recorded from the sensor and standard deviation, measured at different pH and Zn2+ concentrations. Acknowledgement S.M. thanks the Council for Scientific and Industrial Research (CSIR), India for research fellowship, C.S. acknowledges funding from the Department of Science and Technology (DST)-Nanomission program (SR/NM/TP- 56/2016) and the Indian Institute of Technology, Bombay for infrastructural support. References 1. Bandodkar, A. J.; Jeerapan, I.; Wang, J. Wearable Chemical Sensors: Present Challenges and Future Prospects. ACS Sens. 2016, 1 (5), 464-482, DOI 10.1021/acssensors.6b00250. 2. Kenry; Yeo, J. C.; Lim, C. T. Emerging flexible and wearable physical sensing platforms for healthcare and biomedical applications. Microsystems & Nanoengineering 2016, 2, 16043, DOI 10.1038/micronano.2016.43. 3. Moronshing, M.; Subramaniam, C. Scalable Approach to Highly Efficient and Rapid Capacitive Deionization with CNT-Thread As Electrodes. ACS Appl. Mater. Interfaces 2017, 9 (46), 39907-39915, DOI 10.1021/acsami.7b11866. 4. Zhou, Y.; Xu, Z.; Yoon, J. Fluorescent and colorimetric chemosensors for detection of nucleotides, FAD and NADH. Chem. Soc. Rev. 2011, 40(5), 2222-2235, DOI 10.1039/c0cs00169d. 5. Shipway, N.A.; Katz, E.; Willner, I. Nanoparticle Arrays on Surfaces for Electronic, Optical and Sensor Applications. ChemPhysChem 2000, 1, 18-52, DOI 10.1002/14397641(20000804)1:13.0.CO;2-L. 6. Gao, W.; Nyein, H. Y. Y.; Shahpar, Z.; Fahad, H. M.; Chen, K.; Emaminejad, S.; Gao, Y.; Tai, L.-C.; Ota, H.; Wu, E.; Bullock, J.; Zeng, Y.; Lien, D.-H.; Javey, A. Wearable Microsensor Array for Multiplexed Heavy Metal Monitoring of Body Fluids. ACS Sens. 2016, 1 (7), 866- 874, DOI 10.1021/acssensors.6b00287. 7. Zhao, D.; Guo, X.; Wang, T.; Alvarez, A.; Shanov, VN.; Heineman, W. R. Simultaneous Detection of Heavy Metals by Anodic Stripping Voltammetry using Carbon Nanotube Thread. Electroanalysis 2014, 26 (3), 488-496, DOI 10.1002/elan.201300511.


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For Table of Contents use only: This work depicts a ultra-sensitive, specific, rapid and mechanically flexible electrochemical sensing platform for detection of Zn2+ from soil (for scientific farming) and perspiration (for medical diagnostics)


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Point-of-Care, Cable-Type Electrochemical Zn2+ Sensor with

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