Development of poly(safranine-co-phenosafranine)/GNPs/MWCNTs nanocomposites for quartz crystal microbalance sensor detection of arsenic (III) ions

Contamination of drinking water by heavy metals is extremely dangerous to human health. The formation of a quartz crystal microbalance (QCM) sensor for the rapid and portable detection of harmful heavy metals such as arsenic (As) ions in water samples is detailed in this work. Equimolar ratios of safranine (SF) and phenosafranine (Ph) copolymers (PSF-Ph) were synthesized via a chemical oxidative polymerization approach. The copolymer was modified with multi-wall carbon nanotubes (MWCNTs) and graphene nanoplatelets (GNPs) at different percentages (1, 3, 5, and 10%) to form nanocomposites of PSF-Ph/MWCNTs/GNPs. Thermal analysis of the nanocomposites revealed that the final polymer decomposition temperature (PDTfinal) values fell between 619 and 630 °C, and the nanocomposite with 10% loading exhibited the highest decomposition temperatures for T10, T30, and T50. The nanohybrid QCM sensor detected As(III) down to parts-per-billion levels based on the change in the oscillation frequency. The sensor was tested on water samples spiked with different concentrations of As(III) (0–20 ppb). A strong linear correlation (R2 ≈ 0.99) between the frequency shift and concentration with a low detection limit (0.1 ppb) validated the quantitative detection capability of the sensor. This QCM platform with an optimal recognition ligand is a promising field-deployable tool for on-site arsenic analysis in water.


Introduction
Trace heavy metals such as arsenic (As) are extremely toxic environmental contaminants present in water bodies owing to their natural occurrence or anthropogenic activities [1].Prolonged exposure to even low concentrations of these metals can have chronic health effects, including cancer and cardiovascular and neurological disorders [2].The World Health Organization has set a maximum permissible limit of 10 ppb for As(III) in drinking water [3].However, many countries still report higher levels, which calls for rigorous monitoring [4].Conventional analytical techniques for quantifying heavy metals, such as atomic absorption spectrometry (AAS), inductively coupled plasma-mass spectrometry (ICP-MS), and inductively coupled plasma-optical emission spectrometry (ICP-OES), are expensive, time-consuming, and require centralized laboratory infrastructure [5].There is a need for portable, rapid, and cost-effective sensors with parts per billion (ppb) sensitivity for on-site analysis to safeguard against heavy-metal toxicity.
Quartz crystal microbalance (QCM) is a highly effective, affordable, and expeditious sensing technique in the nanogram range for detecting trace heavy metal ions in aqueous solutions.The sensing process employs a piezoelectric quartz crystal that changes its resonance frequency upon the adsorption of mass, which can be precisely measured using a QCM resonator [6].The mechanism of this technique involves measuring the alterations in resonant frequency that occur owing to the formation of affinity complexes in the sensing area [7].
The use of custom-designed receptor materials to modify quartz crystal electrodes has significantly expanded the range of properties and applications of chemical sensors [8].To optimize the functional performance of sensing applications, it is crucial to develop an efficient immobilization strategy that provides high sensitivity and stability [9].Several sensors have been designed to specifically identify cationic heavy metals in aqueous environments by incorporating affinity functional groups with electron-rich systems into the material network.These modified sensors, such as conductive polymers, yield electrostatic interactions with the target cations and cause them to adsorb onto the surface of the QCM electrode [10][11][12][13][14].
Conductive polymers of aromatic diamines, such as benzidine, naphthidine, and phenylenediamines, and their derivatives, have garnered attention in the field of oxidative polymerization [15].Owing to their large conjugated π-systems, these polymers are expected to possess high conductivity, thermal stability, and mechanical strength [16,17].One class of conductive polymers, including water-soluble dyes such as methylene dye derivatives, has commonly been exploited as catalysts to promote the oxidation or reduction of organic molecules [18][19][20].Among these dyes, safranine is a red, phenazine-type dye with an electron donor group (NH 2 ) and a wide heterocyclic conjugated system.Safranine, phenosafranine dyes and their polymeric form are widely employed in various studies, including the pharmaceutical industry, biological, and electrochemical sensors [21][22][23].However, there is limited research on the electrochemical properties of polymeric safranine [21].Although some studies have investigated the electrochemical applications of poly(safranine) in the sensor domain using the electrochemical polymerization film method, only a few articles are available on chemical oxidative polymerization [24].
One of the most promising upgrades to enhance the sensitivity, reliability, and response time of sensors has been achieved through the application of nanotechnology to material properties [25].This innovation overcomes the limitations of conventional materials, and has led to significant advancements.Nanomaterials that exhibit properties that differ significantly from those of higher-structural-dimension materials have been produced in the nanoscale range of 1-100 nm.Carbon-based materials, one of the most studied and currently used materials in nanotechnology, possess extraordinary physicochemical properties that make them cost-effective alternatives to electronic compounds [26].Carbonaceous structures offer numerous advantages over other commonly used materials, including their excellent performance and environmental friendliness.Consequently, carbon nanostructures have been investigated for use in powerful sensor devices owing to their superior physical and chemical parameters, which yield high-quality sensing properties [27][28][29].
Carbon nanotubes (CNTs) possess distinctive characteristics resulting from their high surface-to-volume ratio and hollow structures.These qualities make CNTs ideal candidates for the production of chemical and biological sensors, as they are capable of absorbing a considerable number of molecules onto their surfaces through electronic interactions.Additionally, CNTs exhibit exceptional electrical properties, including high carrier mobility, near-perfect quantum efficiency, and ultrathin network [30][31][32].
Graphene and its derivatives, such as graphene nanoplatelets (GNPs), have been widely modified with different polymers for sensors owing to their exceptional properties, including high thermal and electrical conductivity, large surface area, rapid electron transfer rates, and mechanical strength [33,34].Modern investigations have demonstrated noteworthy outcomes for graphene-based sensors, including enhanced sensitivity, user-friendly operation, rapid response and recovery times, and selectivity [35][36][37][38].
Furthermore, the interaction between compounds that form hybrid materials often leads to synergistic effects, resulting in the unique properties of the developing material owing to the complementary nature of the individual features of each compound [39].Hybrid materials based on carbon structures between MWCNTs and GNPs have been extensively studied for analytical applications due to their excellent combination characteristics, including a highly delocalized π-conjugation system with high surface area, mechanical stability, adaptability, and functionality [40,41].Therefore, a hybrid of MWCNTs and GNPs was fabricated with conductive polymers for sensing applications to improve the conductivity and electro transfer, surface area, and thermal stability of the sensor.The MWCNTs, GNPs hydrides, and polymer network in this scenario exhibit van der Waals forces and ππ stacking interactions, as well as mechanical entanglement between molecules [42][43][44][45][46][47][48].
This paper reports the development of a novel QCM sensor using poly(safranine-co-phenosafranine)/MWCNTs/ GNPs nanocomposites as sensing ligands for trace determination of As(III) in water.To the best of our knowledge, this is the first report investigating a copolymer of safranine and phenosafranine as a sensor for arsenic ions.

Instrumentation
Fourier transform infrared (FT-IR) spectra were recorded using a PerkinElmer (Spectrum 100 FT-IR, US) device in the range 4000-500 cm −1 .The morphologies and elemental distributions of the polymers were examined by scanning electron microscopy (SEM, TESCAN VEGA 3, Czech Republic).The samples were mounted on aluminum microscopy stubs using carbon tape and then coated with gold (Au) for 120 s using Quorum techniques Ltd., sputter coater (Q150t, UK).Transmission electron microscopy (TEM) (ThermoFisher Scientific, Multi-purpose, Talos F200i S/TEM, US) operating at 200 kV was used to investigate the interactions between the polymers and nanomaterials for high-resolution imaging and analysis applications.X-ray diffraction (XRD) data for the designated materials were collected using a Philips x-ray unit (Philips generator PW-1710, Holland) diffractometer with a Cu-Kα irradiation source.The range of 2θ = 5°-80°was scanned at a rate of 1°/min.A thermogravimetric analyzer (TGA-Iris 209 Netzsch, Germany) was used to study the thermal stability with a sample size of (11-15 mg), quality control (calcium oxalate monohydrate (CaOx), artificial air (10 ml/min O 2 , 40 ml/min N 2 ), temperature range of 20C-1000 °C, and heating rate of 10 °C min −1 .QCM resonators (10 MHz) with gold electrodes (OpenQCM, Italy) were used.An Orion Star A111 benchtop pH meter (Thermo Scientific, US) was used to determine the pH of the solutions.

Synthesis of poly(safranine-co-phenosafranine) copolymer (PSF-Ph)
In this study, safranine and phenosafranine were copolymerized via a chemical oxidative polymerization process in acidic media (Scheme 1).The bare copolymers of poly(safranine-co-phenosafranine) were prepared in equal ratios (1:1) using the original chemical oxidative polymerization process of poly(safranine) [24].In two necks round flask, safranine (1 mmol) and phenosafranine (1 mmol) were oxidized with ammonium persulfate (NH 4 ) 2 S 2 O 8 (3 mmol) in 0.2 M of hydrochloric acid.The oxidant solution was added dropwise to safranine and phenosafranine mixtures at room temperature.The reaction mixture was stirred for 24 h under a N 2 atmosphere.A dark red precipitate was suspended in a colorless aqueous medium and formed from the initial red solution.The solids were separated by filtration, rinsed thoroughly with 0.2 M hydrochloric acid and water, and dried for 48 h.

Preparation of poly(safranine-co-phenosafranine)/GNPs/MWCNTs nanocomposites (PSF-Ph/GNPs/ MWCNTs)
The bare copolymer was modified with GNPs and MWCNTs at different loadings to enhance the electrochemical properties of the nanocomposite (Scheme 1).A new series of nanocomposites, PSF-Ph/GNPs/ MWCNTs-1%, PSF-Ph/GNPs/MWCNTs-3%, PSF-Ph/GNPs/MWCNTs-5%, and PSF-Ph/GNPs/MWCNTs-10%, was prepared by applying in situ oxidative polymerization under ultrasound irradiation.Prior to the fabrication process, a mixed G/MWCNTs (1:1) hybrid nanofiller composite was prepared according to a previously literature [49,50].The fabrication process followed the nanocomposite series of poly(safranine-cophenosafranine)/GNPs/MWCNTs through an in situ chemical oxidative polymerization technique using different loadings of equimolar ratios of the GNPs/MWCNTs mixture.Under ultrasound radiation, 1%, 3%, 5%, and 10% of the GNPs/MWCNTs mixture with respect to the unmodified copolymers was employed for 30 min in 0.2 M HCl.For each formulation, the two monomers were added in equal ratios, followed by continuous sonication for 30 min.The reaction was then oxidized with a solution of ammonium persulfate (NH 4 ) 2 S 2 O 8 (3 mmol) at room temperature to initiate the polymerization process, and the reaction mixture was stirred for 24 h under a N 2 atmosphere.The formed precipitates were separated by filtration, rinsed with 0.2 M hydrochloric acid, and dried for 48 h to produce the designed nanocomposites Ultrasound has been employed in diverse areas of nano-chemistry and nanotechnology.Ultrasonic irradiation is a simple, eco-friendly, and multipurpose synthetic tool for creating nanostructured materials that are difficult to access using conventional methods.When applied to liquids, ultrasonic irradiation generates pressure waves that result in the formation of cavities, which can be harnessed in the process of homogenization to fully distribute the nanoparticles in the final product [51].

Immobilization of poly(safranine-co-phenosafranine)/GNPs/MWCNTs nanocomposites on QCM electrode
The QCM was used to evaluate the PSF-Ph/GNPs/MWCNTs nanocomposites as arsenic sensors.The gold electrode surfaces were initially cleaned by immersion in boiling acetone and 50 mM KOH/25% H 2 O 2 for 5 min, rinsed with deionized water, and dried under nitrogen.Next, the electrochemical cleaning process was carried out by scanning the potential between −0.3 and 1.6 V in 0.5 M H 2 SO 4 solution until a stable cyclic voltammogram profile was achieved.The polymer film deposition involved the preparation of a 0.1 mM solution of the PSF-Ph/GNPs/MWCNTs nanocomposite in a 50:50 ethanol:water mixture.The cleaned gold electrode was immersed in this solution for 24 h at room temperature to provide sufficient time for polymer attachment.
After immersion, the electrodes were removed from the modification solutions, thoroughly rinsed with deionized water to remove loosely bound polymers, and then dried under nitrogen gas.Finally, the polymermodified electrodes were spin coated at 2000 rpm for 60 s using a Laurell WS-650SZ-6NPP/LITE spin coater with 0.05 mM PSF-Ph/GNPs/MWCNTs nanocomposite solution to form a uniform thin film over the previously deposited polymer layer.The rotation speed was set to 2000 revolutions per minute (rpm) for 1 min.The spin coating process improved the consistency and stability of the nanocomposite films.The completed electrode modification involved an initial adsorption-based deposition step, followed by mechanical stabilization of the film using spin coating.This dual procedure resulted in a smooth and robust nanostructured interface for selective and sensitive As(III) binding in the QCM measurements.To prepare the heavy metal solutions, various concentrations of As(III) (0-20 ppb) were prepared by diluting the stocks in ultrapure water/ 0.5% HNO 3 .The samples were freshly prepared on the day of analysis.

QCM measurements
A QCM resonator is installed in the instrument cell.The solutions were pumped through the cell at a flow rate of 30 μl min −1 , and the frequency shift was recorded continuously.Baseline stabilization with a blank was followed by analyte exposure for 5 min each.

Results and discussions
The copolymer nanocomposites were studied and analyzed using different characteristic tools.Fouriertransform infrared spectroscopy (FT-IR) was used to investigate the obtained structure, and the thermal behavior was investigated using thermogravimetric analysis (TGA) and Derivative Thermogravimetry (DTG).Surface morphologies were analyzed using Scanning Electron Microscopy (SEM), transmission electron microscopy (TEM) and x-ray diffraction (XRD).

FT-IR study
In an attempt to study and identify the furnished nanocomposites, FT-IR was examined for both the pure copolymer and modified nanocomposites, resulting in multiple fundamental absorption frequencies.Figure 1 shows the IR spectra of both safranine and phenosafranine in comparison with the obtained copolymer PSF-Ph.In the spectra of both monomers, the NH 2 group appeared as two broad peaks at 3457 and 3325 cm −1 (figures 1(a) and (b)).The conversion of the two bands in the spectra of the monomers to one broad band at 3319 cm −1 in the spectrum of the copolymer PSF-Ph is due to the protonation of the nitrogen group (figure 1(c)) assigned to the NH, which confirms the formation of the polymer chain [24,52].Furthermore, the vibration of (=C-H) symmetric aromatic stretching bands is located between 3055-3120 cm −1 in the monomers spectra, while the corresponding characteristic peaks transform to a broad peak at 3090 cm −1 , verifying the higher intensity of (=C-H) stretching resulting from the formation of the copolymer backbone.The aliphatic (C-H) group appeared at 2854-2923 cm −1 in the safranine spectrum (figure 1(a)), which was attributed to the presence of the (CH 3 ) group, and the band disappeared in the phenosafranine spectrum (figure 1(b)).Moreover, a band reappeared in the copolymer spectrum at 2848-2929 cm −1 (figure 1(c)).The bands at 1570 and 1628 cm −1 in the monomer and co-monomer spectra (figures 1(a) and (b)) credited to (C=C) symmetric and asymmetric aromatic ring stretching displayed a broad band with a slight shift to 1639 cm −1 and 1599 cm −1 in the PSF-Ph spectrum (figure 1(c)).The (C-N) stretching bands at 1289 and 1301 cm −1 (figures 1(a) and (b)) changed to a broad band between 1266 and 1326 cm −1 in the copolymer spectra, assigned to the π-electron delocalization induced in the polymer through protonation or C-N-C stretching vibration.The (C=C) and (C-N) stretching modes of vibration were assigned to the quinonoid and benzenoid units of PSF-Ph in a similar way to that of poly(aniline) [53][54][55].
The FT-IR spectra of the PSF-Ph/GNPs/MWCNTs-1%, PSF-Ph/GNPs/MWCNTs-3%, PSF-Ph/GNPs/ MWCNTs-5%, and PSF-Ph/GNPs/MWCNTs-10% nanocomposites (figure 2) were nearly identical to those of pure PSF-Ph with a moderate shift.The nanocomposite spectra showed a new peak at 1528 cm −1 , ascribed to the skeleton vibration (C-C) of the MWCNTs and GNPs surfaces [53,56].Moreover, the bands at 1295 and 1156 cm −1 were assigned to the quinonoid and benzenoid rings and the aromatic (C-H) bending became more distinct in the formation of the nanocomposites.The FT-IR characterization of the copolymer and its nanocomposites revealed their successful formation, although the nanocomposite spectra showed a broader range owing to the interaction between the GNPs/MWCNTs and the polymer blend [57].

Morphology study
The surface morphologies of the new copolymer and nanocomposites series were studied using scanning electron microscopy (SEM) at different magnification between 3kx to 10kx and a scale bar from 10 to 2 μm (figure 3).The SEM images of the unmodified copolymer PSF-Ph (figures 3(a)-(c)) display an asymmetrical fragmented morphology comprising small and large particles.Some of the fragments showed an uneven spherical aggregated shape in the absence of nanostructures [24].Furthermore, the morphology of the nanocomposite PSF-Ph/GNPs/MWCNTs-10% (figures 3(d)-(g)) exhibits the dispersion of GNPs and MWCNTs on the surface and interior of the nanocomposite agglomerate.MWCNTs display as a winding thin linear and formed a bridge between copolymer particle aggregates that uniformly distributed as shown in (figures 3(d) and (e)), the linking species of MWCNTs to the copolymer and GNPs contributed to the enrichment of sensor signal.GNPs possessed a relatively smooth planar structure with scattering particles and comparatively thin layers, as shown at higher magnification (figure 3(f)).The incorporation of the MWCNTs and the copolymer backbone with GNPs created a thicker layer on the surface (figure 3(g)) [52,[58][59][60][61][62].Moreover, the nanocomposites images depict the utilization of multilayered structures, which presented a wrinkled, rough, and well-defined topography, to establish suitable sensor structures for QCM arsenic detection.High-magnification SEM analysis demonstrated the crucial role of the network/layer structure of PSF-Ph/GNPs-MWCNTs-10% on gold electrodes in facilitating efficient electron transfer between the modified electrode and arsenic solutions, ultimately leading to improved electrochemical performance [63].
Energy-dispersive x-ray (EDX) analysis of the pure PSF-Ph and the nanocomposites PSF-Ph/GNPs/ MWCNTs-10% (figures 4(a) and (b)) display the elemental composition spectra and the peaks corresponding to C, N, O, and Cl.The EDX mapping of the pure copolymer and PSF-Ph/GNPs/MWCNTs-10% (figures 4(c) and (d)) were studied to clarify the elemental distribution on the polymers model.Evidently, the elements C, N, O, and Cl are homogeneously dispersed on the copolymer and nanocomposites matrix.It is worth noting that the Au peak detected in the EDX spectra is related to the Au coating deposited on the samples before analysis.The results of the SEM and EDX analyses, which were conducted on the copolymer and its nanocomposite synthesized through in situ oxidative polymerization, were found to be consistent with the successful synthesis.
More detailed evidence of the fashioned copolymers and nanocomposites microstructures were provided by the TEM images in figure 5.The TEM micrographs of the pure copolymer PSF-Ph and the nanocomposite PSF-Ph/GNPs/MWCNTs-10% were compared.The pure copolymer PSF-Ph exhibited aggregation with an irregular morphology, characterized by stacking of multiple copolymer layers (figures 5(a) and (b)).Conversely, the micrograph of the PSF-Ph/GNPs/MWCNTs-10% nanocomposites (figures 5(c)-(f)) showed hollow tubulars structure of MWCNTs with some dark fragments.The GNPs (figures 5(c) and (d)) appear as many folds with lighter color, representing stacking of graphene layers by π-π interactions among layers, plausibly increasing the contact surface area between the GNPs, MWCNTs, and copolymer [58][59][60][61][62].It seems that the distribution of the nanomaterials within the copolymer was uniform.Micrographic analysis indicated that the GNPs aggregated in clusters as a result of van der Waals forces, which caused the MWCNTs and GNPs to bind together, forming a connected and reinforced structure that was part of a continuous chain [63][64][65].The incorporation of MWCNTs and GNPs resulted in stable interactions between the nanofillers and PSF-Ph copolymers facilitated by donor-acceptor interactions, which ultimately enhanced the electrochemical performance of the resulting nanocomposites by increasing both mass and electron transfer [63,66,67].
The loading of nanoparticles on the polymer network and the crystalline nature of the bare copolymer and nanocomposites were studied using x-ray diffraction (figure 6).The crystallographic patterns of PSF-Ph and its nanocomposites illustrated a predominantly semi-crystalline microstructure.The XRD spectra showed several diffraction peaks in the area between 7°and 25°corresponding to the perpendicular and periodicities parallel to the polymer chains as well as intermolecular π-π stacking of the polymer matrix [17,68,69].Moreover, the XRD patterns of the PSF-Ph/GNPs/MWCNTs-1%, PSF-Ph/GNPs/MWCNTs-3%, PSF-Ph/GNPs/MWCNTs-5%, and PSF-Ph/GNPs/MWCNTs-10% nanocomposites exhibited a basal reflection (002) peak at 26°assigned to the d spacing between graphitic walls, which gradually increased with increasing percentage of GNPs/MWCNTs from 1 to 10%.The low-intensity diffraction peaks at approximately 53°and 75°are attributed to the (004) and (110) diffraction patterns of typical graphite [58,59,70].The presence of graphite patterns with increasing intensity confirms the penetration of the GNPs/MWCNTs into the copolymer network.These findings are consistent with the results of the thermal analyses, which indicated an improvement in thermal stability upon the addition of GNPs/MWCNTs.

Thermal study
To gain a more comprehensive understanding of the potential applications and thermal performance of the synthesized PSF-Ph and nanocomposites, thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG) were performed over a temperature range of 25 °C-1000 °C at a heating rate of 10 °C min −1 , as shown in figures 7(a)-(e) and 8(a)-(e).To the best of our knowledge, the TGA/DTG behavior of poly (safranine) or poly (phenosafranine) has not been investigated, but modified safranine with triglycine acetate has been studied [71].The thermogram curves of both the bare copolymer and nanocomposites exhibited overlapping steps that could be distinguished into three weight loss steps.The first stage befallen between 25 °C and 120 °C, which may be related to the loss of moisture, followed by the second stage between 222 °C and 432 °C.The main degradation step of the polymer matrix (PDT main )occurred rapidly from 432 °C to 653 °C, starting at 30% thermal decomposition and embracing PDT max .The hybridization of the copolymer with nanofiller GNPs/MWCNTs in the nanocomposites PSF-Ph/GNPs/MWCNTs-1%, PSF-Ph/GNPs/MWCNTs-3%, PSF-Ph/GNPs/ MWCNTs-5%, and PSF-Ph/GNPs/MWCNTs-10% showed a similar thermal performance (figures 7(b)-(e)).A slight enhancement was observed for the nanocomposites in comparison with that of the bare copolymer, indicating the advantageous effect of the nanomaterial on the thermal stability of the polymers [61,68,70,72].The thermal decompositions at 10%, 30%, and 50% are summarized in table 1.The values of T 10 , T 30 , and T 50 Figure 2. FT-IR spectra for the copolymer PSF-Ph and PSF-Ph/GNPs/MWCNTs-1%, PSF-Ph/GNPs/MWCNTs-3%, PSF-Ph/ GNPs/MWCNTs-5%, and PSF-Ph/GNPs/MWCNTs-10% nanocomposites.revealed a moderately dependent performance pattern that increased with the addition of the GNPs/MWCNTs.The final and maximum temperatures of polymer decomposition (PDT final ) and (PDT max ) are outlined in table 1 based on the TGA and DTG curves, respectively.As shown in table 1, the PDT final values fell between 619 and 630 °C, while PDT max values were in the range of 541 °C-565 °C.PSF-Ph /GNPs/MWCNTs-10% exhibited the highest decomposition temperatures for T 10 , T 30 , and T 50 , along with both PDT final at 630 °C and PDT max at 565 °C.The results obtained from the PDT max and PDT final values confirm the higher thermal stability of the designed nanocomposites materials in the order PSF-Ph/GNPs/MWCNTs-10% > PSF-Ph/ GNPs/MWCNTs-3% > PSF-Ph/GNPs/MWCNTs-5% > PSF-Ph/GNPs/MWCNTs-1% > PSF-Ph.

QCM investigation
The sensing of arsenic ions on the surface of the PSF-Ph/GNPs/MWCNTs hybrids was measured and observed over time at 25 °C and pH 3.5 using QCM.QCM measurements for sensing As(III) ions were performed at pH 3.5 based on solution speciation considerations that maximize As(III) existing as uncharged arsenious acid (H 3 AsO 3 ).This neutral form demonstrated the strongest binding affinity to the PSF-Ph/GNPs/MWCNTs nanocomposite sensing layer compared to the negatively charged arsenic acid species that exist at higher pH levels [73].Additionally, the polymer nanocomposites showed greater stability under acidic conditions, whereas As(III) remained non-volatile.Therefore, pH 3.5 was determined to be optimal for both binding and sensor performance.The pH was adjusted using HNO 3 and NaOH solutions as required and monitored using a pH meter.Figure 9 shows the QCM response upon testing different As (III) concentrations in ultrapure water.The frequency steadily decreased with increasing As(III) concentration as more mass was deposited on the PSF-Ph/ GNPs/MWCNTs nanocomposites-coated electrode surface.The sensor showed excellent linearity over a wide dynamic range with R 2 = 0.99 between frequency shift and As(III) concentration (figure 9).The direct masssensing capability of QCM enables quantification based on concentration-dependent frequency changes.The distinct frequency shifts demonstrate the selective detection of As(III) ions by affinity binding to the respective PSF-Ph/GNPs/MWCNTs nanocomposites.The optimized polymer interface imparts specificity as well as enhances sensitivity.As illustrated in figure 9, the introduction of carbon-based hybrid nanofillers containing graphene nanoplatelets into the copolymer network enhanced the detection of As(III) ions due to the high surface area of the nanoscale polymer layer, as follows: PSF-Ph/GNPs/MWCNTs-10% > PSF-Ph/GNPs/ MWCNTs-5% > PSF-Ph/GNPs/MWCNTs-3% > PSF-Ph/GNPs/MWCNTs-1% > PSF-Ph, which facilitates greater analyte capture.Portable QCM instrumentation allows convenient on-site operation.However, the adsorption of As(III) ions onto the surface of the PSF-Ph/GNPs/MWCNTs nanocomposites was evaluated in nanograms.
To convert the frequency change observed in a Quartz Crystal Microbalance (QCM) to mass in nanograms, 1.The resonant frequency f 0 ( ) of the bare QCM crystal, which was the baseline frequency, was recorded before deposition.2. The sample is deposited on the QCM crystal surface and the new resonant frequency f ( )is noted.
3. the frequency change was calculated using the equation The Sauerbrey equation was used to relate the frequency change to the deposited mass: ( ) is the sensitivity constant of the crystal (ng cm −2 Hz −1 ), f ( ) D is the measured frequency change (Hz), and n ( ) is the oscillation frequency (MHz).
5. For a 10 MHz AT-cut QCM crystal, the sensitivity constant C is approximately 0.177 ng cm −2 Hz −1 6.The C value and measured f D were substituted to calculate the mass change Δm in nanograms.
Nevertheless, the Sauerbrey equation converts the recorded frequency shift into an equivalent mass change by relating the two via the sensitivity constant.This allows for the quantitative determination of nanogram amounts of the material deposited on the QCM sensor.Quartz Crystal Microbalance (QCM) data demonstrate the excellent sensing capabilities of all PSF-Ph/GNPs/MWCNTs nanocomposites for arsenic ions across various experimental concentrations.The concentration range tested ranged from 1 to 20 parts per billion (ppb), and the QCM method successfully detected As (III) ions at all corresponding concentrations.This suggests that the PSF-Ph/GNPs/MWCNTs copolymers exhibit high sensitivity and accuracy in detecting As(III) ions.Additionally, regardless of the concentration, all samples achieved successful sensing within approximately 3 min.These findings highlight the reliable and efficient performance of the PSF-Ph/GNPs/MWCNTs in detecting As(III) ions across a wide range of concentrations in a short time.The consistent and rapid sensing capabilities of these samples further enhance their applicability in various scientific and environmental monitoring contexts.
The lowest detectable concentration based on the minimum resolvable frequency shift was estimated to be 0.1 ppb As(III).Integration with microfluidics can further improve detection limits.The simple sensing protocol, involving sample injection and real-time monitoring, enables rapid quantification within minutes.The proposed QCM methodology offers a promising platform for on-field and in-line heavy metal detection in water.

Sensor performance evaluation
The sensor exhibited excellent accuracy and selectivity for arsenic (As) detection across the tested concentration range of 0.1-20 ppb.A strong linear correlation (R2 ≈ 0.99) was observed between the frequency shift and the As(III) concentration (figure 9), indicating the precise quantification of As(III) ions.The low detection limit (0.1 ppb) satisfies the typical requirements for trace analysis.The accuracy of the PSF-Ph/GNPs/MWCNTs hybrid nanocomposites in detecting varying concentrations of arsenic was evaluated by calculating the percentage recovery from spiked water samples.Known concentrations of As(III) were added to ultrapure water samples and analyzed using the PSF-Ph/GNPs/MWCNTs copolymers-coated QCM sensors.The frequency shifts measured by the sensors were converted to As(III) mass values using the Sauerbrey equation.These experimentally obtained values were compared with the expected concentrations to determine the percentage recovery, as follows:

Recovery
Measured concentration Expected concentration % 100 ( ) / = Five replicate samples were tested at As(III) spiking level of 0.1 ppb.The average percentage recoveries ranged from 96.5% to 99.9% across all concentrations (table 2 and figure 10).The excellent recovery rates, close to 100%, indicate highly accurate quantification of As(III) ions using the developed PSF-Ph/GNPs/MWCNTs sensor with no consistent over-or under-estimations.The precision was also high, with standard deviations of 2.5% in the measurements from the five replicates.Furthermore, the control experiments with blank samples without As(III) spiking showed a negligible sensor response, confirming the absence of false-positive results.However, the QCM platform with PSF-Ph/GNPs/ MWCNTs nanocomposites sensing layer demonstrated excellent accuracy for As(III) quantification in water through precise recoveries, high precision between measurements, no false positives, and selective analyte binding.High selectivity was achieved by using the PSF-Ph/GNPs/MWCNTs copolymers to create a surface interface optimized for As(III) capture.The control experiments with blank solutions showed negligible frequency shifts, confirming the selective binding of arsenic to the PSF-Ph/GNPs/MWCNTs layers.
The selectivity of the developed PSF-Ph/GNPs/MWCNTs sensor was evaluated by testing solutions containing potential interfering ions along with arsenic.Solutions with a fixed arsenic concentration of 10 ppb were prepared by spiking the pure water samples.Various interfering ions, including Cd 2+ , Pb 2+ , Hg 2+ , Ni 2+ , and Zn 2+ , were then added individually at 10 times higher concentration (0.1 ppb).The mixed solutions flowed over the PSF-Ph/GNPs/MWCNTs-coated QCM sensor, and frequency shifts were measured.The frequency change observed for each mixed solution was expressed as percentage ratio relative to the shift caused by 0.1 ppb As(III) alone.As shown in the bar graph (figure 11), negligible responses were observed for all interfering ions, in which a decrease in Δf value refers to sensing flow by an increase in v value by almost the same value of decrease, which indicates full leaching of ions from the sensor surface compared to ∼100% for the arsenic control.This selectivity is attributed to the specific chelating mechanism between As(III) and the rich electron system of PSF-Ph/GNPs/MWCNTs interface.This high selectivity eliminates the possibility of overestimating As(III) due to the presence of accompanying contaminants.It also prevents false-negative results by enabling unambiguous arsenic detection, even in complex water matrices containing diverse background species.Thus, the PSF-Ph/ GNPs/MWCNTs-QCM sensor provides a reliable screening tool for accurate analysis in real field applications.
The reproducibility of measurements across five independently fabricated sensors was also evaluated using 0.1 ppb test solutions.A low relative standard deviation (RSD) of 2.5% was obtained for the frequency shifts.This reliable reproducibility enables the mass production and commercial adoption of the developed sensors.Furthermore, the results of the repeatability study demonstrated that the QCM sensor exhibited consistent performance and a high degree of stability throughout the multiple cycles of the test, as observed by the frequency shift-time curve (figure 12).Moreover, the sensor displayed a high level of reliability and reproducibility, suggesting its potential use in various applications.In addition, The QCM platform with tailored PSF-Ph/GNPs/MWCNTs interface demonstrated highly accurate and selective quantification of As(III) across environmentally relevant concentrations.Its precise yet portable detection capability makes it suitable for on-site heavy-metal analysis.Integration with microfluidics and multiplexing using sensor arrays can further improve the performance of practical applications.
The performance of the QCM sensor was evaluated by comparing it to other studies on As 3+ ion sensors that utilized QCM and other methods (table 3).The table shows the modified materials used and achieved detection limits.As demonstrated in the table, the As 3+ ion sensor developed in this study exhibited a lower detection limit than other reported QCM studies.

Conclusion
Selective detection of As(III) traces in water was achieved through the effective development of a QCM-based heavy metal sensor using PSF-Ph/GNPs/MWCNTs nanocomposites polymers.An oxidative polymerization method was used to create new safranine and phenosafranine copolymers.The thermal and electrochemical properties of the copolymer were enhanced by the addition of GNPs/MWCNTs with varying loadings.A semicrystalline surface and excellent heat stability are characteristics of the engineered nanocomposites.The order of thermal stability of the designed nanocomposites materials is found to be as follows: PSF-Ph/GNPs/ MWCNTs-10% > PSF-Ph/GNPs/MWCNTs-3% > PSF-Ph/GNPs/MWCNTs-5% > PSF-Ph/GNPs/ MWCNTs-1% > PSF-Ph.The nanocomposite images show multilayered structures with wrinkled, rough, and well-defined topographies, which are used to create QCM arsenic detection sensors.High sensitivity toward As(III) ions was observed.The optimized nanoscale interface on the transducer surface allows for a sensitive and rapid analysis using portable instrumentation.The direct mass-sensing mechanism facilitated quantification over a wide dynamic range with excellent linearity.Sensor technology has the potential for extensive applications in on-site environmental and industrial monitoring of toxic heavy-metal contamination.

Scheme 1 .
Scheme 1. Systematic illustration of the chemical oxidative polymerization of the bare copolymer PSF-Ph and the fabrication of PSF-Ph/MWCNTs/GNPs nanocomposites using MWCNT and GNPs in acidic media.

Figure 9 .
Figure 9. QCM sensorgram curve of PSF-Ph/GNPs/MWCNTs nanocomposites showing response to different As +3 concentrations at pH value 3.5 for all samples.

Figure 11 .
Figure 11.Sensorgram curves of As(III) and all interfering ions of PSF-Ph/GNPs/MWCNTs nanocomposites showing Sens of As ions than all interfering ions.
a Values were determined by TGA at a heating rate of 10 °C min −1 .b Values were determined from the DTG curves.

Table 2 .
The evaluation of PSF-Ph/GNPs/MWCNTs nanocomposites sensor accuracy for Sens 0.1ppb of As(III) ion at pH value 3.5.

Table 3 .
Performances comparison of the present QCM sensor with other reported As 3+ detection.