Introduction

Point-of-care testing provides rapid analyte detection close to the patient, which facilitates an improved diagnosis, monitoring, and management of different diseases. Currently, the early screening, diagnosis, and monitoring of cancer require expensive, time-consuming, and sophisticated devices that require high expertise personnel and centralized laboratories. For early screening and cancer biomarker detection, new devices are required for point-of-care diagnostics [1,2,3,4].

Sarcosine, also known as N-methylglycine, is a byproduct and intermediate in the synthesis and degradation of glycine. It is considered a potential biomarker of prostate cancer [5, 6]. The concentration levels of sarcosine range from 2 to 20 µM in the urine of prostate cancer patients based on the cancer progression state, which may reach the metastasis stage in some cases [7, 8] Moreover, the high levels of sarcosine have been connected to various diseases such as dementia, Alzheimer’s, dimethylglycine dehydrogenase deficiency, sarcosinemia, eosinophilic esophagitis, and Lewy body disease [9]. Different methods have been reported for sarcosine determination, including enzymatic colorimetric or fluorometric assays [10,11,12], HPLC with fluorescence detection [13], LC-MS [14, 15], GC-MS [16], and capillary electrophoresis [17]. However, these methods have some major disadvantages, such as the need to additional time-consuming sample pre-treatment or derivatization steps, expertise in operating the instruments, and the high cost of enzymes and instrumental setup. To overcome these drawbacks, electrochemical biosensors are considered attractive alternative analytical tools for fabricating point-of-care diagnostics [9]. They are highly selective, sensitive, rapid, inexpensive, and simple to use and handle, which allows early diagnosis and management of diseases, therefore facilitating timely therapy decisions [1].

One of the most important classes of electrochemical sensors is potentiometric sensors that transduce the activity of a certain dissolved analyte into an electrical potential. However, only a few potentiometric sensors based on molecularly imprinted polymers or sarcosine antibody were reported for sarcosine determination [18,19,20].

The conventional liquid contact sensors, in which the ion-selective membrane (ISM) is inserted between the inner filling and the sample solutions, show certain disadvantages such as fragility, the possibility of the flux of primary ion from the inner filling solution into the sample, and the risk of leakage or drying of inner filling solution. Accordingly, the elimination of inner filling solution in SC sensors overcomes these drawbacks by offering good rigidity, durability, and compatibility with miniaturization [21]. Moreover, screen-printed electrodes show extra advantages over other classical solid-contact electrodes, as being disposable, inexpensive, and versatile. However, SC sensors may suffer from potential drift and a prolonged response time because of the high charge transfer resistance at the interface between ISM and SC. Besides, the water layer formed between the SC and the membrane acts as an electrolyte reservoir which re-equilibrates with each sample composition change leading to a long response time. In addition, diffusion of atmospheric gases as CO2 or O2 through the ISM is favoured by this aqueous layer. A change in pH at the interface or redox side reactions may occur because of CO2 or O2, respectively, causing potential drifts. Moreover, the water layer may decrease the sensor lifetime via detachment of the membrane from the SC [22, 23]. To overcome those drawbacks, different electroactive materials were introduced as ion-to-electron transducer layers to be incorporated between the SC and the ISM.

Several electroactive conducting polymers with mixed ionic and electronic conductivities, as poly(3-octylthiophene) (POT), polypyrrole (PPy), and poly(3, 4-ethlyenedioxythiophene) (PEDOT), were reported to be used as ion-to-electron transducers [24]. PANI is one of the most favourable conducting polymers due to its excellent electrical properties, good hydrophobicity and stability, and ease of preparation from a low-cost monomer [25, 26]. Recently, metal oxides have been introduced as ion-to-electron transducers in potentiometric SC sensors [27, 28]. However, the research in this field is still limited and needs further investigation. WO3 receives considerable attention in photocatalysis, electrochemistry, and phototherapy due to its high chemical stability, excellent electrical conductivity, and good adherence to the substrate [29]. Among approaches to design a robust sensor is the combination of two types of materials to gain the beneficial properties of both materials into one electrode. Nanocomposite materials have attracted attention and have been widely used in many applications due to their synergistic or complementary properties derived from several components [30].

To the best of our knowledge, WO3 NPs and PANI-WO3 nanocomposite have not been investigated before as ion-to-electron transducer layers in potentiometric SC sensors. Thus, the present study describes the chemical synthesis, characterization, and incorporation of WO3 NPs, PANI NPs, and PANI-WO3 nanocomposite as an ion-to-electron transducer in potentiometric SC sensors for the determination of sarcosine in urine. The effect of their incorporation was investigated by comparing it with a control sensor that was designed by excluding the ion-to-electron transducer layer.

Experimental

Chemicals and solutions

Sarcosine, glycine, urea, creatinine, sodium tungstate dihydrate, calix-4-arene, high molecular weight polyvinyl chloride (PVC), 2-nitrophenyl octyl ether (o-NPOE), sodium tetraphenylborate (NaTPB), and tetrahydrofuran (THF) were obtained from Sigma-Aldrich. Sodium dodecyl sulfate (SDS), aniline, ammonium persulfate, and oxalic acid were purchased from Alfa Aesar. Boric acid, phosphoric acid, acetic acid, hydrochloric acid, and sodium hydroxide were obtained from El-Nasr Company. Sarcosine solutions (1 × 10−2 to 1 × 10−14 M) were prepared using Britton-Robinson buffer (pH 5.0).

Twenty-four-hour urine samples were freshly obtained from a healthy male volunteer, 35 years old, and were diluted using Britton-Robinson buffer (pH 5.0).

Instrumentation

Particle size was estimated by dynamic light scattering (DLS) using a Malvern Zetasizer Nano-ZS (Malvern, UK) equipped with a 4-mW He-Ne laser operating at a wavelength of 633 nm. Surface morphology was studied using a Quanta FEG 250 scanning electron microscope (SEM) (FEI, USA) with accelerating voltages ranging from 200 V to 30 kV and magnifications ranging from 20 × to 1,000,000 × . The X-ray diffraction (XRD) analysis was carried out using a PANalytical X’Pert diffractometer using CuKα1 radiation at 45 kV and 40 mA (Malvern, UK). Fourier transform infrared (FTIR) spectra were recorded using a Vertex 80 V FTIR spectrometer (Bruker, Germany). UV-VIS spectrophotometric measurements were performed using a 1601 PC UV-Visible double-beam spectrophotometer (Shimadzu, Japan). To evaluate the electrochemical properties of the developed layers, electrochemical impedance spectroscopy (EIS) was conducted using a potentiostat/galvanostat PGSTAT204 with Nova 11.1.1 software (Metrohm Autolab, Switzerland). A three-electrode configuration was used consisting of a screen-printed sensor with the studied layer, 3-mm diameter (CH Instruments, USA) as the working electrode, an Ag/AgCl electrode (Metrohm, Switzerland) as the reference electrode, and a Pt wire as the counter electrode. Impedance spectra were recorded in 0.1 M KCl over a frequency range from 100 kHz to 100 mHz using an AC amplitude of 50 mV at the open-circuit potential. A digital ion analyzer model no. 3330 (Jenway, UK) with an Orion 900200 Ag/AgCl double junction reference electrode containing 3 M KCl as a filling solution (Thermo Fisher Scientific, USA) was used for potential measurements.

Synthesis of WO3 NPs

WO3 nanoparticles were prepared by a hydrothermal approach using 100 mL of an aqueous solution of 0.125 M sodium tungstate dihydrate. Then 3.2 g of oxalic acid was added to the solution with continuous stirring for 30 min. The hydrochloric acid was slowly dropped into the solution with continuous stirring for 15 min to keep the pH of the solution in the range of 1–2. The solution volume was completed up to 250 mL using double distilled water. Seventy milliliters of the prepared solution was transferred into a 100-mL Teflon-lined hydrothermal autoclave. The autoclave was sealed and kept in an oven at a temperature of 180 °C for 21 h, then cooled down to room temperature. Finally, WO3 NP powder was filtrated and washed sequentially with distilled water and ethanol several times to remove impurities, then dried at 60 °C.

Synthesis of PANI NPs

Micellar emulsion chemical polymerization was used for PANI synthesis [31] using SDS as a surfactant. Equimolar quantities (1.3 M) of SDS and aniline were added to 100 mL of water in a thermostated bath at 20 °C with continuous stirring for 1 h. After that, 100 mL of 1.3 M ammonium persulfate was added dropwise. The polymerization process continued for 2.5 h. The dark green dispersion formed was purified using a 12,000-Da dialysis membrane (Sigma-Aldrich) against deionized water for 48 h. Eventually, the dispersion was washed with water and centrifuged at 15,000 rpm for 10 min four times. The collected sediment dried at 60 °C for 24 h.

Sensor’s fabrication

The dispersions of ion-to-electron transducer layers were prepared as follows:

  1. 1.

    WO3 NP dispersion: 10 mg of WO3 NPs was dispersed ultrasonically in 1 mL of THF.

  2. 2.

    PANI NP dispersion: 10 mg of PANI NPs was dispersed ultrasonically in 1 mL of THF.

  3. 3.

    PANI-WO3 nanocomposite dispersion: 10 mg of PANI NPs and 10 mg of WO3 NPs were dispersed ultrasonically in 1 mL of THF.

Ultrasonic dispersion was performed using the ultrasonic bath model CP500D (42–45 kHz) (Crest Ultrasonics, USA) for 30 min. Fifteen microliters of each dispersion was drop-cast separately on a commercial screen-printed electrode of 3-mm diameter (CH Instruments, USA). The solvent was allowed to evaporate before adding 30 µL of the ISM solution (32.82% PVC, 66.60% NPOE, 0.16% NaTPB, and 0.42% calix-4-arene dissolved in 6 mL THF).

For comparison, a control sensor was prepared by applying the ISM directly on the screen-printed electrode without an ion-to-electron transducer layer. The four sensors were preconditioned by immersing in 10−2 M sarcosine solution for 24 h.

Results and discussion

Material characterization

The mean hydrodynamic particles sizes of the prepared PANI NPs, WO3 NPs, and PANI-WO3 nanocomposite were measured by DLS using a zetasizer and found to be 158.5 nm, 146.2 nm, and 222.4 nm, respectively.

The surface morphology of PANI, WO3 NPs, and PANI-WO3 layers was investigated by SEM (Fig. 1a–c). The scan of PANI revealed an irregular, rough surface (Fig. 1a, a*). The SEM image of WO3 NPs showed a broccoli-like structure that was constructed by several small rectangular nanoplates with a high surface area, as clearly observed in the magnified image (Fig. 1b, b*). The nanocomposite scan illustrated that WO3 NPs were embedded and uniformly distributed within the PANI matrix (Fig. 1c, c*). The rough morphology and high surface area can significantly influence signal transduction.

Fig. 1
figure 1

SEM images of PANI NPs (a, a*), WO3 NPs (b, b*), and PANI-WO3 nanocomposite (c, c*). * Images are of higher magnification

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The crystalline structure of PANI, WO3, and PANI-WO3 nanocomposite was analyzed by XRD as presented in Fig. 2. It can be clearly observed that the XRD patterns of PANI exhibited a broad and low intensity peak at 2θ = 20–30°, which is revealed by its amorphous nature [32]. In contrast, WO3 showed sharp crystalline peaks corresponding to the highly crystalline hexagonal structure of the h-WO3 phase (JCPDS card no: 00-033-1387) with a preferred orientation (200) plane [33]. On the other hand, the XRD pattern of PANI-WO3 nanocomposite shows the same diffraction peaks fitting to the hexagonal h-WO3 phase with lower intensities compared to the pure hexagonal h-WO3, which might be attributed to the distribution of WO3 NPs within the amorphous PANI matrix [32].

Fig. 2
figure 2

XRD patterns of PANI NPs, WO3 NPs, and PANI-WO3 nanocomposite

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The FTIR spectra show the in-plane and out-of-plane different mode vibrations of molecules or atoms with associated energies in the IR region (Fig. 3). The main PANI characteristic peaks are assigned as follows: the peaks at 3340 and 3230 cm−1 can be attributed to the free (non-hydrogen bonded) N–H stretching vibration and hydrogen-bonded N–H bond between amine and imine sites. The bands at 1565 and 1486 cm−1 are related to the C = N and C = C stretching vibration modes for the quinonoid and benzenoid units. The bands at 1290 and 1232 cm−1 are assigned to the C–N stretching mode of benzenoid units, while the band at 1112 cm−1 to quinonoid unit. The peak at 793 cm−1 is attributed to C–H out of plane bending vibrations of the benzenoid unit [34]. In the FTIR of WO3, the broad band in the range of 600–800 cm−1 represents W-O-W stretching modes in WO3 crystal [35]. The FTIR spectrum of PANI-WO3 nanocomposite showed the main peaks of pure PANI combined with peaks of WO3 with a small shift and lower intensities, indicating the formation of a PANI-WO3 matrix as confirmed by XRD measurements.

Fig. 3
figure 3

FTIR spectra of PANI NPs, WO3 NPs, and PANI-WO3 nanocomposite

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Figure 4 shows the UV-VIS absorption spectrum of PANI dispersion in the conducting form (emeraldine salt). At 785 nm, it revealed localized polaron bands (π—polaron transition). At 420 nm, it showed (polaron—π* transition) and at 350 nm (π – π* band). In alkaline pH, the PANI characteristic absorption spectrum indicates the conversion of PANI NPs from the conducting emeraldine salt to the insulating emeraldine base. A shift from 350 to 320 nm was observed in the polaron band of the π – π*. Moreover, a peak at 560 nm appeared, which is assigned to the exciton band of base form [31]. In the case of PANI-WO3 dispersion, the absorption spectrum showed the same characteristic peaks of PANI with higher absorbance due to the introduction of WO3 [36].

Fig. 4
figure 4

UV-VIS spectra of PANI NPs in the ES form and after dedoping to the EB form using 0.5 M NaOH, WO3 NPs, and PANI-WO3 nanocomposite

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Electrochemical characterization

EIS measurements were conducted in 0.1 M KCl solution in the frequency range of 100 kHz to 100 mHz, using Randles’ equivalent circuit (Fig. S1). EIS measurements in the high-frequency region represent bulk membrane resistance (Rbc) in conjunction with contact resistance between the electronic conductor and the ISM. The geometric capacitance value (Cg) can also be obtained from the high-frequency region while the low-frequency region represents double-layer capacitance (Cdl) together with charge-transfer resistance (Rct) at the interface between the electronic conductor and the ISM. This data provides indications about the ease of ion-to-electron transfer at the electrode interface [37]. As shown in Table S1, the nanocomposite PANI-WO3-based sensor showed the lowest Rbc and Rct, while its Cg and Cdl were the highest. This could be attributed to the synergistic effect of both PANI and WO3 NPs when combined together. Besides the outstanding electrochemical properties of the conductive polymer PANI, the addition of WO3 NPs with their high surface area, as proved by SEM, and their high capacitance to PANI matrix resulted in a nanocomposite with improved capacitance and lower resistance. As a consequence, this should provide easier ion-to-electron transfer at the electrode interface between the ISM and the underlying SPE with a stable potentiometric response.

Potentiometric response

The performance of WO3 NPs, PANI NPs, and PANI-WO3 nanocomposite-based sensors was evaluated compared to a control sensor free from an ion-to-electron transducer layer in batch mode. The potentiometric responses were measured in sarcosine solutions prepared in Britton-Robinson buffer (pH 5.0) in the range of 10−2–10−14 M. The calibration curves of the four proposed sensors are presented in Fig. 5. The limit of detection (LOD) was calculated from the intersection of the two extrapolated linear segments of the calibration curves. The potentiometric performance characteristics of the four proposed sensors are summarized in Table 1.

Fig. 5
figure 5

Profile of the potential (mV) versus -log concentrations of sarcosine (M) of the proposed sensors. The standard deviation of three measurements (n = 3) is represented by the error bars

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Table 1 The potentiometric performance characteristics of the four proposed sensors
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From Table 1, it can be clearly seen that the inclusion of WO3 NPs, PANI NPs, and PANI-WO3 nanocomposite as ion-to-electron transducers improved the performance of the sensors compared to the blank sensor in terms of linearity range, detection limit, response time, lifetime, potential drift, and sensitivity [28, 38, 39].

The practical response time of the four proposed sensors measured along the linearity range of each sensor at pH 5.0 (Fig. 6).

Fig. 6
figure 6

Potential-time curve of the proposed sensors at pH 5.0: a blank sensor, b WO3-based sensor, c PANI-based sensor, and d PANI-WO3-based sensor

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Potential stability

To investigate the potential stability of the sensors, the emf was recorded over 2 h. A small drift of 0.8, 0.7, and 0.5 mV h−1 was obtained for WO3, PANI, and PANI-WO3 nanocomposite-based sensors, respectively. The blank sensor showed the highest drift (10 mV h−1). The improvement in potential stability observed after the addition of WO3, PANI, and PANI-WO3 nanocomposite is attributed to decreased resistance and increased capacitance, allowing easier ion-to-electron transfer at the sensors interface, which is consistent with EIS results.

The lifetime (long potential stability) of the proposed sensors was found to be 2.5, 3, and 4 months for WO3, PANI, and WO3-PANI-based sensors, respectively, compared to 2 months for the control sensor.

The longer response time, higher potential drift, and shorter lifetime shown by the control sensor may be attributed to the formation of a water layer between the SC and ISM. In order to evaluate this, a potentiometric water layer test was performed.

Potentiometric water layer test

The test was performed by measuring the potential drift upon changing from a primary ion solution (sarcosine; 10−7 M) to a highly concentrated interfering ion solution (glycine; 10−3 M) and then back to the primary ion solution (sarcosine; 10−7 M). The water layer that may be formed below the ISM causes the diffusion of gases and ions through the membrane. Changes in pH due to diffusion of CO2 and/or diffusion of O2 result in redox side reactions that consequently lead to a drift in potential. As the water layer may behave as a reservoir for ions that need re-equilibration upon each change in the analyte composition, longer response times are observed. Besides, the formation of this water layer may cause rapid ISM detachment from the SC, which dramatically affects the sensor lifetime [22, 23].

As shown in Fig. S2, the presence of the proposed ion-to-electron transducer layers improved the stability of sensors and decreased the potential drift to 0.7 mV h−1, 0.8 mV h−1, and 0.5 mV h−1 for WO3, PANI, and PANI-WO3-based sensors, respectively. However, in the case of the control sensor, where no ion-to-electron transducer layer was used, a layer of water was formed below the ISM resulting in a high potential drift of 10 mV h−1. In addition to PANI’s outstanding properties in ion-to-electron transduction, the highly hydrophobic character of PANI could reduce the formation of the aqueous layer [40]. Moreover, the high surface area offered by WO3 NPs could provide strong adhesion between the SC and the membrane.

Effect of pH on sensor performance

The effect of pH on the sensor’s response was investigated using Britton-Robinson buffer over the pH range of 2–11. As shown in Fig. S3, the four sensors showed constant potentials over the pH range of 2–6. Accordingly, this range was chosen as a working pH range for the sensors. The pK1 and pK2 of sarcosine are 2.98 and 8.55, respectively. Therefore, at different pH values, sarcosine exists in different forms [41]. At acidic pH, sarcosine is mainly in its protonated form, which may enhance attraction with the anionic sites of NaTPB within the PVC membrane. However, at higher pHs, sarcosine molecules start to acquire a negative charge, causing repulsion with tetraphenyl borate and consequently decreasing the response [42].

This data also indicates that the ISM prevents the transition of the pH-sensitive PANI layer from emeraldine salt form to base form. As previously reported by Lindfors and Ivaska [26], this can be explained by the repulsion between OH− ions in the solution and the anionic NaTPB within the membrane. They also reported that bulky doping anions such as SDS or DBSA can suppress PANI pH sensitivity instead of small ones such as Cl− [43].

Sensor’s selectivity

Urine is an aqueous solution of greater than 95% water, with a minimum of these remaining constituents as urea 9.3 g/L, creatinine 0.670 g/L, chloride 1.87 g/L, sodium 1.17 g/L, and potassium 0.750 g/L. Separate solution method was used to investigate the potentiometric selectivity coefficients of the proposed sensors for structurally related compounds such as glycine. Selectivity towards components that may be present in urine, such as creatinine, urea, uric acid, and proteins (albumin, gamma globulin, and alpha-1-acid glycoprotein), was investigated.

Log ϰpot sarcosine, glycine =  − 0.77, − 3.79, − 2.37, and − 2.00, while Log ϰpot sarcosine, creatinine =  − 1.18, − 4.34, − 2.64, and − 3.50, Log ϰpot sarcosine, urea =  − 0.34, − 3.82,—− 2.86, and − 3.36, Log ϰpot sarcosine, uric acid =  − 1.17, − 4.33, − 2.63, and − 3.49, Log ϰpot sarcosine, albumin =  − 0.33, − 3.79, − 2.83, and − 3.33, Log ϰpot sarcosine, gamma globulin =  − 1.15, − 4.23, − 2.57, and − 3.41, and Log ϰpot sarcosine, alpha-1-acid glycoprotein =  − 1.09, − 4.04, − 2.46, and − 3.26 for control sensor and WO3, PANI, and WO3-PANI-based sensors, respectively.

The selectivity obtained may be attributed to the selective complexation with the ionophore. The interaction between calixarene host and guest molecules to form an inclusion complex is primarily governed by the size of the calixarene cavity, which determines the selectivity and binding strengths of these receptors to different guest molecules according to size-fit factor. Sarcosine may form an inclusion complex via its N-methyl group deeply embedded in the ionophore cavity. This structural arrangement allows the stabilization of the complex by CH-π contacts between the N-methyl residue and the aromatic pocket, showing higher selectivity over structurally related glycine as reported by Biavardi et al. [44]. Moreover, owing to the lipophilicity of sarcosine compared to hydrophilic ions such as Na+, K+, and Cl− or the less hydrophobic glycine, they are incapable of replacing sarcosine in the ion-selective membrane.

Direct determination of sarcosine in urine

For determination of sarcosine in urine using blank, WO3-based, and PANI-based sensors, 5 mL of urine was transferred into 50-mL volumetric flasks separately, and different volumes of sarcosine standard solution (10−2 M) were added and completed to the volume with Britton-Robinson buffer (pH 5.0). Regarding the PANI-WO3-based sensor, a higher dilution ratio is used so that the clinical range of sarcosine in urine would be covered by the concentration range of the proposed sensor. Accordingly, 100 µL of urine was transferred into 50-mL volumetric flasks separately, spiked with different volumes of sarcosine standard solution then completed to the volume with Britton-Robinson buffer (pH 5.0). The potentiometric response was recorded. The percentage recoveries obtained by the proposed sensors are demonstrated in Table S2. The accurate and precise percentage recoveries obtained confirmed the ability of the proposed sensor to be applied for sarcosine determination in urine without the need for preliminary extraction, purification, or derivatization steps.

Compliance of the PANI-WO3 point-of-care (POC) diagnostic sensor with WHO ASSURED criteria

Following the WHO guidelines, our proposed sensor meets the “ASSURED” criteria for POC diagnostics [45] as being: (1) affordable: the screen-printed electrodes as a substrate are an inexpensive platform compared to other costly instrumentations used in other techniques. Moreover, PANI and WO3 are synthesized from cheap precursors; (2) sensitive: the PANI-WO3-based sensor showed extreme sensitivity with a LOD 9.95 × 10−13 M. The other sensors showed also low detection limits of 9.84 × 10−8, 9.79 × 10−9, and 9.54 × 10−10 M for blank, WO3, and PANI-based sensors, respectively, which could easily reach sarcosine concentration in urine; (3) selective: the sensor can detect sarcosine quantitatively in the presence of its interferants, which would reduce false positives; (4) user-friendly: the proposed sensors are simple to use; (5) rapid: the sensors showed a rapid response time ranging from 10 to 20 s. Moreover, they were successfully applied for direct sarcosine determination in urine without the need for additional time-consuming sample pre-treatment or derivatization steps; (6) equipment free: on-site data analysis requires only small portable equipment. In contrast to other commonly used techniques that require complicated equipment, (7) deliverable/disposable.

The proposed sensors in this work were compared to the previously reported electrochemical sensors in literature for sarcosine determination as summarized in Table 2. It is worth mentioning that our WO3-PANI-based sensor showed superior sensitivity over the other reported sensors with very short response time.

Table 2 Comparison of this work with reported sarcosine electrochemical sensors
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Conclusion and future directions

Portable, disposable, and cost-effective sarcosine SC ion-selective potentiometric sensors using PANI NPs, WO3 NPs, and PANI-WO3 nanocomposite as ion-to-electron transducers were fabricated and compared to transducer-free control sensor. For the first time, WO3 NPs or PANI-WO3 nanocomposite have been used as ion-to-electron transducer layers in potentiometric SC sensors. The inclusion of PANI and WO3 in SC sensors enhanced the ion-to-electron transduction at the interface between the screen-printed sensor and the ISM offering more stable potential responses with lower potential drifts, a faster response time, a longer lifetime, and better sensitivity. The proposed sensors exhibited Nernstian slopes over linear response ranges of 10−3–10−7 M, 10−3–10−8 M, 10−5–10−9 M, and 10−7–10−12 M for blank sensor, WO3 NPs, PANI NPs, and PANI-WO3 nanocomposite-based sensors, respectively. From a comparative point of view between the four sensors, the inclusion of PANI-WO3 nanocomposite offered the lowest potential drift, the longest lifetime (4 months), and the best sensitivity (9.95 × 10−13 M). The proposed sensors proved to be capable to determine sarcosine as a potential prostate cancer biomarker in urine without prior sample treatment steps or using expensive and sophisticated instruments. The analysis could be carried out near the patient without the need to transfer samples to central laboratories. Moreover, they are disposable, rapid, cost-effective, and simple to fabricate and use. Therefore, the proposed sensors could be considered good candidates as point-of-care diagnostics for the rapid diagnosis of prostate cancer.

Searching for new electroactive materials as intermediation layers in SC-ISE has drawn attention for many years. The promising properties of the proposed nanocomposite may encourage upcoming research to investigate other combinations of conducting polymers with metal oxides as ion-to-electron transducers. Further work is needed for microfabrication of the proposed sensors to be integrated with an embedded system providing miniaturized detection device for commercial clinical applications.