Design of a smart sensor mesh for the measurement of pH in ostomy applications
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A carbon-loaded polyethylene film was modified through a combination of laser ablation and electrochemical anodisation to yield a mechanically flexible yet electroanalytically sensitive mesh. A custom flavin derivative bearing a pendant phenol substituent was electropolymerised onto the substrate, and its electrochemical properties were investigated. The reversible flavin electrochemistry was retained (Ep = −0.374 V vs. Ag/AgCl, pH 7), and the peak position was found to shift by 60 mV/pH over a range covering pH 2.55 to pH 8.12. The stability of the resulting composite has been evaluated, and the analytical applicability towards the voltammetric measurement of pH in human urine was critically assessed.
There is an extensive literature on the development of pH sensing systems for monitoring urine pH and covers a wide variety of methodologies such as luminescence imaging , fibre-optic probes , near-infrared reflectance spectroscopy , absorption and fluorescence spectroscopy  and magnetic resonance spectroscopy . Electrochemical techniques, however, predominate [23, 24, 25, 26, 27, 28, 29, 30, 31, 32] and can be more amenable to periodic or continuous monitoring in situ [29, 30, 31, 32]. This is especially relevant when considering the compositional complexity and heterogenous nature (volume, surface area or shape) of the sample matrix. It is only in recent years that there has been an interest in flexible/wearable systems [29, 30, 31, 32] and, in the case of ileostomy and urostomies, there is a need for a mouldable membrane type sensor that is unobtrusive to the patient. The need for device disposability creates further difficulties in the design process where the manufacture of low cost yet robust and reproducible sensing structures is required.
The foundations of the proposed sensor are based on a polyethylene film doped with carbon particles (C-PE) to confer electrically conductivity. The carbon–polymer composite is a commercial variant originally designed to serve as an electromagnetic screening film , but it was envisaged that through removing the outermost polymer layers and exposing a greater proportion of interfacial carbon, the electroanalytical sensing performance could be improved . A mesh design was proposed whereby laser processing could be employed to etch tracks within the film at a level insufficient to penetrate through the film. It could be anticipated that were the tracks rastered across the X and Y planes, a hole would be created where the tracks intersected and thereby effectively create a porous mesh-like structure. While such modifications could be expected to yield a conductive and mechanically mouldable film, it was noted that the underlying carbon would fail to possess any inherent analytical selectivity. Given that the principal aim was to develop a mesh capable of monitoring the pH of bodily fluids, it was therefore necessary to consider a further modification to the base system.
The preparation of the C-PE composite mesh and its subsequent modification with the poly flavin film was therefore investigated, and its potential for use a pH sensing film was critically assessed through examining the response in urine.
All chemicals were obtained from Sigma-Aldrich, were the highest grade available and were used without further purification. Electrochemical analysis was carried out using a VSP-300 Multichannel Potentiostat/Galvanostat/EIS (Bio-Logic Science Instruments, EC-Lab Ltd) with a standard three-electrode configuration with either a glassy carbon or carbon–polyethylene composite as the working electrode. Platinum wire served as the counter electrode and a conventional silver/silver chloride (3 M NaCl, BAS Technicol, UK) reference electrode. All measurements were conducted at 22 ± 2 °C. Britton–Robinson buffers (acetic, boric and phosphoric acids, each at a concentration of 0.04 M and adjusted to the appropriate pH through the addition of sodium hydroxide) were used throughout unless otherwise specified. Butyl-grafted polyethylene films filled with carbon black were supplied by Goodfellow Research Materials (80 μm thick, resistivity of 100 Ω cm).
Preparation/characterisation of modified films
The C-PE film was then sectioned and mounted in a laminate casing with a 4-mm square window and thermally sealed [28, 30]. The latter was done to enable comparative investigation of the film properties. It has become relatively common to electrochemically anodise carbon composite electrodes in order to elicit improved electrochemical behaviour [28, 30]. The electro-oxidation (+ 2 V, 0.1 M NaOH) typically increases exfoliation of the carbon particles—generating more edge plane sites and increases the populations of various oxygen functional groups [28, 30]. This process was also instituted here where it has been found that the unmodified screen-printed electrodes typically exhibit poor electrochemical behaviour with large overpotentials necessary to obtain any significant analytical responses .
Riboflavin was obtained from commercial sources, whereas the phenol derivative was custom synthesised. The synthesis of the 10-(4-hydroxyphenyl)benzo[g]pteridine-2,4(3H,10H)-dione was accomplished using methods previously published . Electropolymerisation was conducted through placing the carbon–polyethylene film into an aqueous solution containing the phenol derivative (150 μM, pH 7). Repetitive scan cyclic voltammetry (+ 0.2 V → − 0.8 V → + 1 V, 50 mV/s) was used to initiate the electropolymerisation process. Solutions were generally degassed with nitrogen prior to commencing the experiments and run under nitrogen blanket.
Analytical characterisation/ethics approval and compliance
The analytical capability of the mesh sensors was assessed using human urine as the test matrix principally to determine the robustness of the material and to assess the possibility of surface passivation from either matrix components or electrode process by-products. Urine was obtained from three healthy volunteers (2M, 1F) who had not taken any form of medication in the 24 h preceding collection of the samples. The samples were collected, anonymised, stored at 4 °C until required and then flushed to waste once the study had concluded. Approval for the use of human urine samples was granted through the Ulster University Ethics Committee (UREC Ref: REC/16/0073) prior to commencing the investigation.
Results and discussion
It was envisaged that the oxidation leads to the formation of oligomeric and polymeric deposits as indicated in Fig. 2. When the scan direction is reversed and the potential is swept back towards the flavin reduction region however, the magnitude of both flavin peak processes is significantly increased. In separate studies involving the phenolic compound, repetitively scanning the flavin region without inducing the oxidation of the phenol did not lead to any change in the peak magnitudes. Thus, the increase in the peak height observed on the second and subsequent scans shown in Fig. 5 can therefore be attributed to the formation of the polymeric deposit on the electrode surface and the accumulation of the material at the interface as anticipated in Fig. 2.
It should be noted that each point is the average of three measurements and although error bars are included within the figure—the variation (typically less than 2 mV) is so small that they are barely discernible from the actual marker. The influence of this repetitive scanning on the peak height can also give some indication of the stability of the flavin film. The normalised peak height (based on the magnitude of the first scan) as a function of scan number is highlighted in Fig. 9b. It can be seen that over 63 repetitive scans (and over 3 entire pH scan series), the oxidation peak associated with the flavin decreases by less than 20%. This is a crucial factor given that the original intention was to engineer a film that could periodically monitor the pH of a biofluid within a device such as a dressing, catheter or stoma where the device would, ideally, need to be in place for several days [1, 2, 3, 4, 5].
The critical point to note from Fig. 8 is that while the peak magnitude decreases (possibly as material is lost from the surface), the peak position remains constant (%RSD = 0.6%; N = 50) throughout both the mechanical flexing and when the film is fixed in a 90° bend. The analytical signal is derived not from the peak magnitude but from the peak position, and thus, the latter’s insensitivity to mechanical flexing is a significant advantage.
Determination of urine pH from healthy volunteers
Commercial probe pH
C-PE/flavin mesh pH
It can be seen that the results are within 0.1 pH unit of the standard system. Repetitive scanning within the urine did not degrade the response and indicates that at least within urine, there is no cumulative fouling of the electrode. The fact that the flavin redox centre can be probed at very low potentials also avoids the possibility of oxidising matrix components such as tyrosine and tryptophan whose products are known to passivate electrode surfaces . The structure of the underlying film is mechanically flexible, and it could be anticipated that it could serve as a versatile film for applications where there is a need to mould the electrode to contours of the prospective sample. While there are a multitude of approaches to measure the pH of urine, the spectroscopic methodologies in the present context are largely unsuitable for direct integration within ostomy appliance. The system exhibits a classical Nernst-like (59 mV/pH) response which is consistent with other solid-state devices employing both potentiometric [18, 19, 20, 21, 22] and voltammetric [25, 26, 27] sensing methodologies. It should be noted however that none of these system have been used in the measurement of pH within biological fluids. The system investigated here provides a number of key advantages over the existing approaches: the flavin redox centre lies in a region where there are few competing processes/interferences, the substrate is flexible and retains performance when bent or folded and the peak is robust. The latter is a critical advantage in that there is very little drift in the measurement of the peak potential—either during normal scanning, after bending or after being fixed at perpendicular angle. The degree of error observed was less than 0.001 pH units, and this was far superior to the potentiometric systems where drift is a common limitation.
The flavin redox film has been shown to be robust to repetitive scanning and can provide an accurate measure of pH in urine. The system is free from common interferences found in biofluids by virtue of the cathodic potentials necessary to interrogate the flavin redox centre, and, critically, the measurement of the oxidation potential can be achieved without the need for degassing. The latter should enable its adoption within a host of smart biomedical/textile applications where there is a need for mechanically flexible but electroanalytically sensitive sensors.
The authors are pleased to acknowledge financial support from the Department of Employment and Learning (DEL) Northern Ireland, EC-Lab Ltd, the British Council (DST-UKIERI: Ref 65/2017) and the University of Central Lancashire Innovation and Enterprise for supporting this work.
AM was involved in ethics approval/human urine analysis and oversight of surface preparation and characterisation. CC contributed to electrochemical characterisation. AM was involved in physical characterisation/modification. CLL and CMT contributed to the synthesis of intermediates, RBS was involved in the supervision and characterisation of synthesis/products, JD contributed to the supervision of overall project and compilation of manuscript.
Compliance with ethical standards
Conflict of interest
The authors confirm that there are no conflicts of interest arising in work presented here.
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