Electrochemical nanosensor for ultrasensitive detection of malachite green and monitoring of its photocatalytic degradation

Herein, we report the synthesis, characterization, and application of TiO2/ZnO and La-doped ZnO nanocomposites for the detection and degradation studies of Malachite Green (MG). TiO2/ZnO and La-doped ZnO nanocomposites were synthesized by reflux and hydrothermal methods, respectively, and characterized by UV–visible spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and energy-dispersive X-ray analysis. A glassy carbon electrode modified with COOH-fMWCNTs and TiO2/ZnO nanocomposite demonstrated high sensitivity characteristics for the sensing of MG up to 0.34 nM limit of detection. The application of a photocatalytic method using 2% La-doped ZnO led to 99% degradation of MG in 40 min. The photocatalytic breakdown of MG followed first-order kinetics as revealed from the spectroscopic and electrochemical monitoring of the degradation process. Color variation offered naked-eye evidence of MG degradation in the specified time. The experimental findings helped in proposing the degradation mechanism. To the best of our knowledge, the current work presents the first example of a novel photocatalyst for almost absolute degradation of MG. Moreover, the electrode modifier as well as the approach adopted is novel and efficient for minute-level detection of MG and monitoring of its photocatalytic degradation.


INTRODUCTION
Water pollution is among the most pressing challenges of this century. Climate change is further aggravating the level of water contamination 1 . This resonates with a rapid surge of industrialization which has led to improved lifestyle but at the cost of an increase in pollution level in different environmental media. According to a recently published report by the UN: Valuing Water, 80% of the untreated industrially polluted water makes its way into water bodies and jeopardizes the efforts for achieving the clean water sustainable development goal. One of the common industrial pollutants discharged into the environment are dyes 2 . A matter of grave concern is that almost one-fifth of the industrial pollution, containing~30% of all the dyes produced globally, is the consequence of the direct release of these dyes into water bodies without any prior treatment 3,4 . Most of the dyes are toxic and pose innumerable risks to the ecosystem 5 . In humans and animals, they can cause mutations, cancers, disruption in hormonal system, allergies, respiratory problems, etc. Moreover, the color of dyes affects the aquatic biota by disrupting the oxygen balance and hindering light penetration, thus, leading to diminished photosynthetic activity.
Among a number of toxic dyes, positively charged malachite green (MG), chemically named as N-methylated diamino triphenylmethane (C 52 H 54 N 4 O 12 ) is extensively used in silk, jute, leather, paper, acrylic, and food industries 6,7 . It has been reported to be carcinogenic and mutagenic 8 . Hence, its presence in water poses serious health risks [9][10][11] . Literature survey reveals that various techniques are in practice for the detection of MG, such as potentiometry 12 , liquid chromatography-mass spectrometry (LC-MS) 13,14 , ultra-performance liquid chromatography with tandem mass spectrometry (UPLC-MS-MS) 15 , surface-enhanced resonance Raman scattering (SERS) 16 , micellar electrokinetic chromatography (MEKC) 17 , enzyme-linked immunosorbent assay (ELISA) 18 , and SERS coupled with high-performance thin-layer chromatography (HPTLC) 19 . Yet there is an urgent need of alternative methods which could be simpler, effective, and fast. Hence, to avoid operational difficulties and tedious sample preparation 20 , electrochemical methods are promising for dye detection due to their fast speed of analysis, high sensitivity, selectivity, cost-effectiveness, and easy handling 21 . Given the robustness and real-time response of the voltammetric techniques, nanosensors are designed for the trace-level detection of water toxins, including MG. This requires modification of the Working Electrode (WE) surface by suitable modifiers that impart sensitivity characteristics to the electrode surface 3,[21][22][23] . To detect MG electrochemically, various modifiers such as multiwall carbon nanotubes-polyethylene immine (MWCNTs-PEI) 24 , silver copper metal organic framework (Ag/Cu-MOF) 25 , CeO 2 /Nafion 26 , MWCNTs 27 , have been reported. In one particular study a somewhat complex platform by immobilizing a nanocomposite film of gold nanoparticles/graphene quantum dots-tungsten disulfide nanosheet (AuNPs/GQDs-WS 2 ) on Glassy Carbon Electrode (GCE) has been documented 28 . This label-free sensitive aptasensor was found to detect MG in fish tissues with LOD of 3.38 nM. However, more research efforts are still required to explore materials for ultrasensitive detection of MG and other toxic dyes by using electrochemical nanosensors. In this regard, the current work explores the use of an effective combination of modifiers for ultrasensitive detection of MG with LOD much better than the sensing platforms as discussed vide infra.
Nanocomposites are the combination of nanomaterials designed to add colossally the benefits of individual materials.
These have attracted great attention of researchers in recent decades and their applications in biomedicine, solar cells, and photocatalysis have been explored extensively [29][30][31][32][33][34] . The most commonly reported titania (TiO 2 )-based nanocomposites are widely employed due to their appealing electrical and structural properties 35 . Zinc oxide also possesses attractive mechanical, electrical and morphological properties 28 . TiO 2 /ZnO nanocomposite when combined with carbon-based nanomaterials such as graphene oxide (GO), CNTs etc., exhibit enhanced charge transfer and electrical properties 35 . The high surface area, electrical, and mechanical properties of CNTs make them suitable for electrode modification 22 . The properties of CNTs are improved by their coiling up into MWCNTs and by chemical functionalization with hydrophilic groups such as carboxylic group 36 . With this consideration, our work explores a novel combination of functionalized multiwall carbon nanotubes (fMWCNTs) and TiO 2 /ZnO nanocomposite for modification of the electrode surface, to make use of their synergistic effects for ultrasensitive detection of MG.
Various chemical and physical methods are in practice for dye removal from wastewater 37 . However, wastewater loaded with dyes poses challenges in terms of its treatment due to the stable and resistive nature of the dyes that refuse to be treated with conventional methods such as precipitation, oxidation, biodegradation etc. 38 . To overcome these challenges, recent decades have seen a surge in the use of photocatalytic methods by employing metal oxides, metal sulfides, oxynitrides and their composites for efficient dye removal [39][40][41] . This method is preferred because it is simple and effective without requiring extreme experimental conditions and high energy 39 . The most striking feature of this method is its reliance on light as a driving force rather than any other fast-depleting non-renewable source. In this regard TiO 2 and ZnO have been at the center stage of photocatalysis. The utility of the photocatalyst depends on certain characteristics such as high surface area, photochemical stability, suitable bandgap, ability to carry out a wide range of redox reactions, long lifetime of charge carriers, low cost, ecofriendliness, etc. No single catalyst fulfills all these requirements but still, some obey most of them. The photocatalyst ZnO offers attractive features like fast charge carrier mobility and transport, ease of crystallization, low cost, eco-friendliness, etc. 42 . It also allows tunability of the bandgap for utilizing a wider region of the solar spectrum. Its performance can be enhanced further by doping 43 . The best photocatalyst should avoid electron-hole pair recombination and work in visible range as sunlight constitutes 46% visible light and only 4-6% UV light. In the present work we have used lanthanum (La) dopant as its doping improves the photocatalytic activity of the catalyst by increasing the binding sites for oxygen and adsorption of hydroxyl ion which acts as a hole scavenger 44 . These features make it a better and a smarter choice under direct sunlight 45 . Therefore, we selected La-doped zinc oxide (La-ZnO) nanocomposite as a photocatalyst for MG degradation.
The present work aims to tackle two dimensions of the problem by synthesizing two types of nanocomposites by exploring their applications for the detection of MG dye in water beyond its permissible threshold and studying its degradation to harmless products to make water safe for human and aquatic life. The novelty of this work is the designing of an ultra-sensitive electrochemical platform prepared via modifying the surface of GCE with fMWCNTs and TiO 2 /ZnO nanocomposite that is not only useful for the nano-level detection of MG but also for monitoring its photocatalytic degradation. The designed electrochemical sensing platform offers additionally an unconventional analytical approach, i.e., drop-casting of the analyte/sample directly on fMWCNTs and TiO 2 /ZnO nanocomposite modified GCE surface. This approach has two peculiar advantages viz. very small sample of the analyte/sample can be examined and closer accessibility of the dye molecules to the electrode surface is achieved as witnessed by the generation of an intense electrochemical signal, which is normally not observed for slowly diffusing large-sized molecular compounds (such as dyes) dissolved in solution. Hence, the adopted approach at the designed nanosensor holds great promise for the trace-level detection and treatment of dyecontaminated wastewater.

RESULTS AND DISCUSSION
Characterization of TiO 2 /ZnO nanocomposite Figure 1a depicts the UV-vis characterization of the as-synthesized TiO 2 /ZnO nanocomposite. The synthesized nanocomposite dispersed in deionized water exhibits a high absorption peak at 371 nm. Using Touc plot, the bandgap of the synthesized nanomaterial was calculated. The UV-vis spectra of the pristine ZnO and TiO 2 NPs show peak at 355 and 300 nm, respectively (see Supplementary Fig. 1). For pure ZnO energy gap value was calculated to be 3.21 eV while for TiO 2 it is 3.08 eV as seen in Supplementary Fig. 1. From Fig. 1b, the energy gap for TiO 2 /ZnO nanocomposite was determined to be 2.98 eV which is lower than the pristine TiO 2 and ZnO.
In the Fourier transform infrared (FTIR) spectrum ( Fig. 1c) of TiO 2 /ZnO sample, it can be seen that C-H stretching of the aromatic ring appears at 3012 cm −1 . The absorption band at 400-1000 cm −1 corresponding to Ti-O-Ti bonding suggests the formation of TiO 2 . The vibrations of Ti-O-Ti are indexed by the band at 1214 cm −1 . The signal at 1730 cm −1 matching with the carbonyl bond clearly demonstrates the non-hydrolytic sol-gel reaction of titanium (IV) isopropoxide (TTIP) with acetic acid to produce the ester 46,47 . This indicates Ti 4+ is bonded to the oxygen of carbonyl bond 48 . The mode of Zn-O stretching is denoted by a sharp band at 512 cm −1 (see ref. 49 ).
The crystallinity and geometry of investigated samples were characterized by XRD. Supplementary Fig. 2 shows the X-ray diffractogram of pure TiO 2 NPs at 2θ values of 25.06°, 37.54°, 48.30°, 54.89°, and 62.59°, which correspond to the (101), (004), (200), (211), and (200) crystal planes, respectively in agreement with JCPDS-00-001-0562. The XRD spectrum of the synthesized TiO 2 /ZnO nanocomposite displayed in Fig. 1d reveals the characteristic peaks of ZnO in the TiO 2 substrate as validated from the JCPDS card number 79-0208 for ZnO. The sharp peak at 34.37°corresponding to the (002) crystal plane relates with the wurtzite structure of ZnO NPs 50 . The Debye-Scherrer formula (Eq. (1)) was used for the calculation of crystallite size: Here, D signifies the particle's crystallite size and K represents the Scherrer constant which has a value of 0.9, λ is the wavelength of light used for diffraction, β shows full width at half-maximum (FWHM) and θ represents the reflection angle. The average crystallite size of TiO 2 /ZnO was estimated to be 23.84 nm. The morphology and elemental composition can be observed in Fig. 1e, f, respectively. The former shows SEM image of TiO 2 / ZnO nanocomposite. TiO 2 NPs have granular irregular morphology and show aggregation of particles. The ZnO nanorods are clearly seen scattered among the TiO 2 NPs. Whereas the EDX spectrum of TiO 2 /ZnO nanocomposite indicates the weight percent of different components of the material. The results reveal that the stoichiometric ratio of Zn and Ti is closer to 1:1. A little off value from 1:1 ratio may be due to the precursor involvement in side reactions. The atomic weight percentages of Ti, Zn, and O are present in identical proportions in the stoichiometric ratios (see Table 1) that confirm the presence of the elements in TiO 2 /ZnO nanocomposite. The results also confirm that there are no impurities in the sample.
T. Imtiaz et al.

Characterization of bare ZnO and La-ZnO
The bare and doped ZnO nanomaterials were characterized by UV-vis, XRD, FTIR, EDX, and SEM analysis. The UV-vis spectra of bare ZnO NPs and La-ZnO nanocomposites can be seen in Fig. 2a. The characteristic absorption peak of ZnO occurs at 355 nm, while in the spectra, 2, 3, and 4% La-doped ZnO NPs peak appears to have shifted to 375 nm, 385 nm, and 388 nm, respectively. The bathochromic effect in the absorption of La-doped ZnO NPs can be related to the bandgap reduction of the nanocomposite. Equation (2) was used for bandgap calculation: Where "α" denotes the absorption coefficient, "h" the photon's energy, "A" the proportionality constant, and n the index.  Fig. 2c(a)) reveals a hexagonal structure 51 . The intense peaks in the X-ray diffractogram suggest that the synthesized ZnO NPs retain their hexagonal wurtzite structure by the addition of La as a dopant. With the increase in concentration of dopant, the strong peak intensified substantially 52 . Within the measurement sensitivity of XRD, peaks associated with the dopant metal oxide or other impurities are conspicuously absent from the examined XRD pattern. Figure 2c demonstrates that the X-ray diffraction peak at 101 plane of the doped samples (La-ZnO nanocomposites) is slightly shifted towards lower Bragg's angle compared to pure ZnO NPs, indicating a larger interplanar distance as compared to pure one. This is attributed to a slight enlargement of the ZnO lattice structure as a result of the greater ionic radius of La 3+ relative to Zn 2+ ion. These observations verify the successful substitution of doped metal oxide into the crystal lattice of ZnO to produce homogenous La-doped ZnO nanocomposites. The crystallite size of bare ZnO NPs and La-ZnO nanocomposites was calculated by using Debye-Scherrer equation as given in Equation 1. The resulting values are presented in Supplementary  Table 1 which shows a clear decreasing trend for the crystallite size as the doping is increased. We were expecting an increase in crystallite radii upon substitution of Zn 2+ (ionic radius = 74 pm) with La 3+ (ionic = 106 pm). However, the crystallite size depends on many factors, one of them being increased nucleation rate in doped material which might be attributed to the decreased particle size. As a general rule, particle size reduces when the nucleation rate is high for a given growth rate and vice versa 53,54 . Figure 2d shows the FTIR spectra of bare ZnO NPs and La-ZnO nanocomposites. The O-H bond stretching of solvent generated a broad peak at 3350 cm −1 . The presence of organic trace residues led to the appearance of signals in the 1700-1000 cm −1 range. The presence or absence of NPs was evident in peaks  below 1000 cm −1 (see ref. 55 ). In case of 2% La-ZnO nanocomposite, the peaks shifting and the appearance of a new small peak at 595 cm −1 indicate La-O bond stretching that reveals the successful doping of ZnO NPs 56 . The structural morphology was probed by SEM imaging of bare ZnO NPs and La-ZnO nanocomposite as shown in Fig. 3. The morphology of the samples at 500 nm magnification reveals that the bare ZnO NPs (Fig. 3a) and La-ZnO nanocomposite ( Fig. 3b) both have irregular spherical (particle-like shape) morpologies. Furthermore, the particle size of the La-ZnO nanocomposite appears somewhat smaller than that of the bare ZnO particles. The EDX spectra of bare ZnO and doped ZnO are provided in Fig. 3c and d which reveal the presence of Zn, O, and La as significant components of the material and provide a quantitative assessment of the compositional elements in weight percentages. The stoichiometric ratios of atomic (weight) percentages for Zn, La, as well as O are presented in proportions as they appear in the sample ( Table 1). The results also show that the sample is free from impurities.

Electrochemical characterization of the modified electrode
The electrochemical characteristics of the modified electrodes were probed by performing electrochemical impedance spectroscopy (EIS) and square wave voltammetry (SWV) to choose the best sensing platform for detection of the target analyte i.e., MG. EIS provides valuable information about the charge transfer characteristics of the designed sensor. EIS parameters of the redox probe (here: 5 mM K 3 [Fe(CN) 6 ]) at the modified surfaces were compared to bare GCE for choosing the best electrode modifier. To carry out EIS investigations, GCE modified with COOH-fMWCNTs and TiO 2 /ZnO and its combination were analyzed using 5 mM K 3 [Fe(CN) 6 ] redox probe in 0.1 M KCl electrolyte with a frequency range from 14 kHz to 1 Hz.
The obtained Nyquist plots of the bare and modified GCEs are shown in Fig. 4a. The diameter of the semicircle is indicative of charge transfer resistance R ct . A large diameter is obtained for bare GCE, whereas the lowest as shown in the inset of Fig. 4a, is for COOH-fMWCNTs with TiO 2 /ZnO modified GCE. This is suggestive of faster charge transduction through the modified electrodes than the unmodified one due to reduction in the interfacial barrier. The R ct for the COOH-fMWCNTs/GCE and (TiO 2 / ZnO)/GCE falling in between these two limits suggest that both of these exhibit higher transduction than the bare GCE but lower than the COOH-fMWCNTs/(TiO 2 /ZnO) modified GCE. The decrease in the R ct is due to fact that the COOH-fMWCNTs and the TiO 2 /ZnO synergistically increase the electroactive surface area as they possess high surface-to-volume ratio and impart enhanced electrical properties. Therefore, the modified matrix results in improving the conductivity of the bare electrode by acting as a bridge between the analyte and the transducer and by providing an increased number of binding positions for the analyte 22 . This also explains the lowest resistance obtained in the case of combination of the aforementioned two modifiers because of their synergistic effects in imparting conductivity characteristics to the transducer 57 . The numerical value of R ct and other parameters were obtained from Randles equivalent circuit presented in the inset of Fig. 4a. The corresponding parameters are enlisted in Supplementary Table 2.
Attributing low R ct and enhanced current response to the increased surface area can be verified by performing cyclic voltammetry (CV) at the bare and modified electrode surfaces. Figure 4b shows the current response of [Fe(CN) 6 ] −3/−4 at bare, (TiO 2 /ZnO) modified, COOH-fMWCNTs modified, and COOH-fMWCNTs/(TiO 2 /ZnO) modified GCEs. The electroactive surface area was calculated by using the For 5 mM K 3 [Fe(CN) 6 ], n = 1, D = 7.6 × 10 −6 cm 2 s −1 , and v = 100 mV s −1 . The equation was employed to find the area A in cm 2 by tapping in the value of peak current I p as obtained from the CVs presented in Fig. 4b. The surface area calculated in all four cases is presented in Supplementary Table 3 that shows a successive increase in the surface area of the electrode from bare to COOH-fMWCNTs/(TiO 2 /ZnO) modified GCE with later being almost five times greater than the former. Thus increased surface area seems to allow faster charge transfer of the redox probe at the modified electrode surface. This trend corroborates well with that obtained from EIS data which allowed us to choose the COOH-fMWCNTs/(TiO 2 /ZnO) modified GCE as an effective sensing platform for further studies.

Voltammetric analysis of the target analyte
The sensitivity and robustness of SWV makes it a useful technique for sensing applications as it is fast and can provide results comparable to other sophisticated techniques such as chromatography and spectroscopy. Therefore, SWV of MG was performed at bare and modified GCE in the potential range of 0 to 1.5 V, with deposition potential and time of −0.3 V and 5 s, respectively. From the voltammograms displayed in Fig. 4c, it is obvious that the peak current at (TiO 2 /ZnO) modified GCE, COOH-fMWCNTs modified GCE, and COOH-fMWCNTs/(TiO 2 /ZnO) modified GCE is threefold, fivefold, and almost sevenfold more intense than the current response obtained at the bare GCE. These results can be attributed to the decreased charge transfer resistance and increased surface area of each electrode as discussed vide supra in the section "Electrochemical characterization of the modified electrode". These findings prompted us to further investigate the figures of merit of COOH-fMWCNTs/(TiO 2 /ZnO) modified GCE as an effective sensing platform. The role of TiO 2/ ZnO nanocomposite can be envisioned as a stepping stone during electrochemical oxidation of the dye molecules and mediator between GCE and COOH-fMWCNTs that enhance the electrochemical oxidation signals. Here, COOH-fMWCNTs are speculated to transfer electron to the GCE from the MG via TiO 2 /ZnO nanocomposite through a hoping mechanism which offers faster kinetics for the transfer of electrons in comparison to direct tunneling electron transfer between MG and bare GCE. It is worth mentioning that the synergistic effect of the components of the sensing platform (COOH-fMWCNTs/(TiO 2 /ZnO)/GCE) for MG gives a significant boost to the oxidation current and reduces the charge transfer resistance by providing more active sites for preconcentrating the analyte that leads to improved sensitivity.

Effect of scan rate
The nature of the process that takes place at electrode's surface was investigated to discern between the diffusion or adsorptioncontrolled process. This was probed by taking cyclic voltammograms at varying scan rates between 25 mV s −1 and 150 mV s −1 using COOH-fMWCNTs/(TiO 2 /ZnO) modified GCE. The resulting voltammograms are shown in Supplementary Fig. 3. The relationship between the scan rate and peak current was used for the assessment of the nature of the process taking place at the electrode/electrolyte interface. The oxidative peak current increased with the scan rate as shown in the inset of Supplementary Fig. 3A. The linearity of the peak current vs scan rate is suggestive of the surface-controlled phenomenon taking place at the COOH-fMWCNTs/(TiO 2 /ZnO) modified GCE.
For the process to be diffusion-controlled, there should be a direct relationship between the I p and v 1/2 . Supplementary Fig. 4 represents the graph between I p and v 1/2 and the information obtained suggests that diffusion-controlled process is not validated at the modified electrode, thus, endorsing the results from the inset of Supplementary Fig. 3A. Thus the confirmation of the adsorption-controlled process comes from the linearity of the plot of log I p versus the log v and comparison of the slope value of the plot shown in Supplementary Fig. 3B with the literature reported values. For the process to be fully controlled by adsorption, the slope should be unity whereas it should be 0.5 for a diffusion-controlled mechanism 21 . In case of our targeted analyte, the plot shown in Supplementary Fig. 3B follows the Eq. (4): The slope value of 0.86 falls in between the two extreme cases as mentioned, which suggests the involvement of both mechanisms, however, with one (adsorption) being dominant over the other (diffusion). Thus, the higher value of correlation coefficient R 2 (0.9973) of the plot between log I p and log v as seen in the inset of Supplementary Fig. 3A than the R 2 (0.9688) of the plot between I p and v 1/2 of Supplementary Fig. 4 provides evidence that adsorption is the dominant controlling mechanism of electron transfer at COOH-fMWCNTs/(TiO 2 /ZnO) modified GCE.

Optimization of experimental parameters
The successful modification of the working electrode characterized by EIS and CV was then followed by recording the response of the analyte using SWV. To get further enhanced SWV sensing response of MG at COOH-fMWCNTs/(TiO 2 /ZnO) modified GCE, several parameters such as supporting electrolyte, pH, deposition potential and deposition time were optimized.
Supporting electrolyte plays a significant role during electrochemical analysis. It can affect the analyte's peak position and current in the voltammograms. Given the crucial role of the supporting electrolyte, various electrolytes were tested for the detection of MG. The acidic (HCl, H 2 SO 4 ), basic (NaOH), neutral (KCl, NaCl), and buffer systems (PBS, BRB) were employed for this purpose. The comparison of the results as shown in Supplementary Fig. 5 shows that the best peak current and peak shape of MG appear in PBS solution. The peak intensity and position obtained in the supporting electrolyte strongly depends on its pH. Therefore, the influence of the pH of PBS on the analyte was probed by recording the voltammograms by varying the pH from 3 to 11. The obtained voltammetric response presented in Fig. 5a shows shift in peak potential and current with changing pH. The highest peak current was noticed in PBS of pH 7. The change in peak position with change in pH of the medium suggests the involvement of proton(s) along with the electron transfer in the electro-oxidation reaction of MG. The information about the number of electrons/proton(s) participating in the system was obtained from the plot of peak potential as a function of pH (Fig. 5b). The information about the electrons involved was obtained by employing Eq. (5) as follows: Where m/n represents the ratio of protons and electrons. The slope value of the plot shown in Fig. 5b is 26 mV which is close to the theoretical Nernstian value (29.5 mV) for a two-electron and one-proton transfer process. After the pH optimization step, the next parameter optimized was the deposition potential. Changing the deposition potential from −0.4 V to +0.5 V changed the intensity of the I p as shown in Supplementary Fig. 6A. The dye being cationic showed an increased current response in the negative window ranging from −0.4 V to −0.2 V and then started declining. This trend can be clearly seen in Supplementary Fig. 6B. The surface saturation at −0.2 V can be related to the maximum interactions between the analyte and the modifier at the GCE surface. Hence, −0.2 V was opted as the optimal accumulation potential. Furthermore, the deposition time also influences the current response of the analyte. Therefore, the performance of the designed sensor was probed by changing the deposition time from 5 to 60 s at −0.2 V deposition potential in PBS supporting electrolytes of pH 7. The recorded voltammograms are presented in Supplementary Fig. 7A. The maximum peak current was achieved at 10 s after which the peak current started decreasing (see Supplementary Fig. 7B). It means that at 10 s all the available sites at the modified GCE are occupied by the analyte molecules with proper orientation. A further increase in deposition time decreased the current response of the analyte, probably by interfering with the correct orientations achieved at the optimal time. Thus, 10 s was chosen as the deposition time for further analytical studies.

Analytical applications of the designed sensor
After the optimization of experimental parameters, the designed sensing platform under optimal conditions was used for the detection and degradation studies of MG dye.
For determination of the limit of detection (LOD), SWV studies of the MG were performed using COOH-fMWCNTs/(TiO 2 /ZnO) modified GCE under pre-optimized conditions of deposition potential (−0.2 V), deposition time (10 s), and PBS (pH 7) as the supporting electrolyte. The current signals obtained at various concentrations of analyte are shown in Fig. 6a. The voltammograms of the lower concentrations are presented in the inset of Fig. 6a. Whereas Fig. 6b represents the calibration plot between the lower concentrations (ranging from 0.01 to 2.75 μM) of MG and corresponding peak current values.
The formulae used for the calculation of the limit of detection and the limit of quantification (LOQ) are given in Eqs. (6) and (7): LOQ ¼ 10 σ=m Where σ represents the standard deviation of the electrodes without drop-casting analyte sample over their surfaces. It was calculated for three and ten samples from their peak values for LOD and LOQ, respectively, while m is the slope of the concentration-current calibration plot. The plot shown in Fig. 6b was used to calculate the LOD and LOQ. The LOD for MG using COOH-fMWCNTs/(TiO 2 /ZnO) modified GCE was evaluated to be 0.34 nM which seems preferable compared to the reported sensors presented in Table 2.

Validity of the designed sensor
The validity of the designed sensor was probed by checking its reproducibility and repeatability. Five different working electrodes were prepared using COOH-fMWCNTs/(TiO 2 /ZnO) recognition layer to study reproducibility. The obtained results shown in Supplementary Fig. 8A exhibited little to no variation, indicating the reproducibility of the designed sensor. For repeatability, using similarly modified working electrodes, voltammograms were scanned after different time intervals. The corresponding results shown in Supplementary Fig. 8B reveal the inter-day stability of the designed sensor. The RSD values of 2.78% (n = 5) for reproducibility and 1.75% for repeatability (n = 7) indicate excellent performance of the designed sensing platform. RSD of less than 3% validates the robustness and stability of the proposed sensor for practical applicability.
In addition, a masking test was used to probe specificity of the sensing platform for ascertaining the validity of the designed sensor for MG. Thus the influence of interfering agents (IAs) on the signal of MG was examined using the designed nanosensor. Real water samples usually contain various contaminants which can interfere with the analyte under investigation. To check the interference of other species on the MG detection ability of COOH-fMWCNTs/(TiO 2 /ZnO) modified GCE, several IAs such as other dyes viz. metanil yellow (MY), methyl red (MR), eriochrome black T (EBT), orange II (Or II), and commonly existing ions of copper, nickel, calcium, zinc, magnesium, sodium, and potassium in water were studied. The IAs were introduced individually into 20 μM solution of MG to check their effect on the peak current response of MG. The results presented in Fig. 7 demonstrate that the presence of these IAs cause no significant variation in the current response of MG under optimal conditions. This is due to the potential-based selectivity of the designed electrochemical sensor and strong binding affinity of the target analyte with the modifier. The peaks of other dyes obtained at different potentials without any substantial effect on the peak current of MG opens another prospect of our sensor, i.e., its ability of detecting other dyes with peaks at different potentials.
Photocatalytic degradation studies of MG After the successful characterization of the synthesized material (section "Characterization of bare ZnO and La-ZnO"), its application in photocatalysis was studied. The photocatalytic decolorization of MG solution was physically observed with naked eyes while UV-vis spectroscopy was used to choose the efficient photocatalyst among bare ZnO NPs and La-ZnO nanocomposites for MG degradation. Four sets of experiments were performed using the ZnO, and 2-4% doped La-ZnO materials, each added to the predetermined concentration (9 μM) of the dye solution. The extent of degradation was monitored by noticing the color variation and its final disappearance and recording absorbance of the solution by UV-vis spectroscopy for 220 min under neutral pH environment. Our studies favored the 2% doped La-ZnO nanocomposite which degraded 92% of the dye present in the solution in the stipulated time. This enhanced photocatalytic activity could be related to the lowering of bandgap due to the presence of 4f orbital of La 58 . Hence, when a photon carrying energy equal to or greater than the bandgap of the semiconductor strikes its surface, an electron from the valance band gets transported to the conduction band, and a hole is created in its place. Consequently, a pair of photogenerated electron and hole is formed. There are two main reactions simultaneously taking place at the surface that help the catalyst avoid undergoing any change during the reaction 59 . These are oxidation reactions that use photogenerated holes and reduction reactions that use photogenerated electrons. The products of these reactions play their unique roles. The oxygen and hydrogen peroxide radicals act as scavengers and prevent the charge recombination process. The hydroxyl radical has a significant contribution to the mineralization of organic pollutants like dyes. These reactions have been summarized in Fig. 8.  Proposed degradation mechanism involves the attack of hydroxyl radical on central carbon of the MG resulting in the cleavage of C−C bond and N,N-dimethylaminobenzyl. Reportedly, p-dimethylamino phenol and 4-(N,N-dimethylamino)methylbenzylone intermediates, the latter being the most common, are formed. Intermediates like benzoic acid, p-(dimethylamino)benzoic acid and hydroquinone are the products of attack of hydroxyl radical on the initially formed intermediates which on ring opening convert to simple organic acid and are mineralized into CO 2 and H 2 O as the final degradation products 58,60 . Figure 8 illustrates the proposed mechanism.

Effect of pH
The photocatalytic efficiency depends greatly on pH of the solution. Therefore, we examined its effect by varying the pH from 3 to 11. For pH adjustment of the solutions, 0.1 M HCL and 0.1 M NaOH were used. By varying the pH, degradation increased from 48 to 99%. The obtained rate constants and extent of degradation are summarized in Supplementary Table  4. The studies at different pH revealed that the dye degraded in all the investigated systems with varying extents of degradation. The rate of photocatalytic degradation increased with the increase in pH of solution, and in basic media more degradation occurred in less time as compared to acidic media. The faster degradation at high pH values is because of the availability of more hydroxyl ions generating hydroxyl radicals at the surface of La-ZnO nanocomposite (see Fig. 8) that accelerate the rate of photodegradation by mineralizing the organic pollutants such as dyes 61 . Resultantly, pH 11 proved to be the optimum value for photocatalytic degradation that degraded 99% of the dye in 40 min.
Monitoring photocatalytic degradation of MG Photocatalytic degradation was performed by adding 2% La-ZnO photocatalyst to a known concentration of MG solution of pH 11. The solution was kept under direct sunlight and subjected to constant stirring for homogenous dispersion of the photocatalyst. The sample was then periodically taken out after every 5 min intervals followed by drop-casting 5 μL of the sample at the surface of COOH-fMWCNTs/(TiO 2 /ZnO) modified GCE. The asprepared nanosensing surfaces were then dipped in PBS of pH 7 for recording the voltammograms. The corresponding voltammetric signatures displayed in Fig. 9A reveal that the current response of the analyte MG decreases with time. This is due to the presence of diminished amount of MG in solution with increasing time due to the ongoing photocatalytic degradation process. Since the current obtained is due to the concentration of the analyte present, hence, the I p value was used to monitor the extent of dye degradation from electrochemical data obtained at the designed nanosensor by using the following Eq. (8): The time-based peak current values of the photocatalytic degradation of MG was used for plotting the curve shown in Supplementary Fig. 9. Kinetic studies were also performed from the time-based peak current diminution data of MG in the presence of photocatalyst. Plotting ln([I p ] t /[I p ] o ) against time gave a straight line as shown in Fig. 9B which suggests 1st-order kinetics of the photocatalytic degradation reaction. The corresponding Eq. (9) is given below: T. Imtiaz et al.
Where, k (rate constant) with a value of 0.117 min −1 was obtained from the slope of the plot shown in Fig. 9B. Spectroscopic technique was also used for monitoring the kinetics of photocatalytic degradation of MG. A known concentration of MG was subjected to degradation in the presence of La-ZnO photocatalyst, and absorbance of the solution was recorded at different time intervals. From the spectra displayed in Fig. 9C, it can be seen that as time passes, the absorbance of MG solution decreases. Since this absorbance gives the measurement of the remaining dye in the system, the information obtained can be used to find the extent of degradation at varying time intervals. The degradation efficiency was estimated according to Eq. (10), as follows: Where C o represents the initial concentration at zero time and C t represents the concentration of the dye at time t where "t" is the irradiation time. By using this formula, Supplementary Fig. 10 was obtained. Kinetic studies were also performed by utilizing the same data. The order of the reaction was established by plotting ln C t /C o as a function of time. The linear plot shown in Fig. 9D offers evidence of the 1st-order kinetics with a rate constant of 0.12 min −1 according to Eq. (11) given below: The rate constant and extent of degradation obtained from both the UV-vis and electrochemical technique shows comparable results. Physical evidence of photocatalytic degradation as it proceeds over time is provided in Fig. 10. So to summarize, the photocatalytic degradation of the dye sample under direct sunlight occurred most efficiently in medium of pH 11. For electrochemical monitoring of the photocatalytic degradation, samples were taken from MG solution of pH 11 (subjected to photocatalysis) after every 5 min and their droplets were used to dry on the designed nanosensor. Whatever amount of the dye has been left after degradation is now dried and available at the surface of nanosensor. So subjecting this timebased left over dye containing nanosensor to electroanalysis by putting it in PBS solution of pH 7 gives signal of the dye. Electroanalysis in PBS of pH 7 was done due to the fact that our sensor has already been proved to give the best response of MG in this medium (see the section "Voltammetric analysis of the  target analyte" and Fig. 5a). Hence, question of compatibility issue of pH 7 and pH 11 does not arise as their solutions are not mixed. The solvent of pH 11 solution is evaporated upon drying the sample droplet at the surface of the designed nanosensor. Degradation is done in solution of pH 11 in a separate flask and the time-based leftover dye in that solution is monitored electrochemically by taking samples from it and putting their droplets on the surface of nanosensor followed by drying and then dipping this dried dye containing nanosenor in the electrochemical cell containing PBS solution of pH 7 followed by recording its voltammogram; the peak height of which tells about the amount of MG left after degradation.
The maximum MG degradation efficiency of 99% was achieved in 40 min in a solution of pH 11 under direct sunlight. The kinetics of photocatalytic degradation monitored by UV-vis spectroscopic method and electrochemical method employing the COOH-fMWCNTs/(TiO 2 /ZnO) modified GCE are in good agreement. Moreover, color variation provides physical evidence of the photocatalytic degradation of MG. The obtained results, thus offer proof of the concept of electrode modification (with suitable modifier for imparting sensitivity characteristics) that efficiently detects MG and monitors its photocatalytic degradation.

Instrumentation
Electrochemical investigations were carried out via Metrohm Auto lab (Utrecht, The Netherlands) with installed NOVA 1.1 software. The cell assembly composed of a 3-electrode system containing Ag/AgCl dipped in saturated KCl solution as the reference electrode, platinum wire as the counter electrode, and bare and modified GCE as the WE. The UV-visible (Shimadzu-1700, Japan) double beam spectro-photometer in 200-800 nm range was used to conduct optical studies of the synthesized materials: TiO 2 , ZnO, TiO 2 /ZnO, and La-doped ZnO nanocomposites. The UV-vis spectroscopic technique was used to record the absorption spectra in order to determine the optical bandgap values of the nanomaterials and nanocomposites. For the examination of crystallinity and measurement of average crystallite size, an X-ray diffractometer (Philips Xpert Pro powder X-beam diffractometer, UK) with Cu Kα radiation was used having a wavelength of 0.1542 nm, an accelerating voltage of 45 kV and a current of 40 mA in 2θ angle ranging between 10 and 80°. The Fourier transform infrared spectroscopy (BRUKER Platinum ATR, Germany) was employed to identify specific functional groups and contaminants in the range of 400-4000 cm −1 . The morphology of the nanomaterials was investigated using scanning electron microscopy (JEOL.JAD-2300 module, Japan) with 5 kV accelerating voltage. The elemental composition of materials was investigated using Energy Dispersive X-ray analysis (EDX) coupled with scanning electron microscopy (SEM). Moreover, SEM (MIRA3 TESCAN) along with EDX was used to study morphology, chemical makeup and size of the as-synthesized nanomaterials.

Synthesis of TiO 2
Sol-gel method was used for the synthesis of TiO 2 NPs. In this typical synthesis, 100 g of TTIP (titanium (IV) isopropoxide) was dissolved in a specific amount of iso-propanol, followed by magnetic stirring for 5 min. As a result, an alkoxide solution was obtained. Subsequently, 127 g of iso-propanol and 25.33 g of water were mixed and added dropwise to the alkoxide solution. The mixture was stirred for 24 h at ambient temperature. Then the as-obtained precipitate of TiO 2 was dried in oven at 100°C which was then calcined in a muffle furnace at 500°C.
Synthesis of TiO 2 /ZnO nanocomposite TiO 2 /ZnO nanocomposite was synthesized by using the reflux method. The synthesis was carried out by mixing 0.548 g of zinc acetate (Zn (CH 3 CO 2 ) 2 ) with 200 mL deionized water and then stirred for 15 min. The as-prepared solution was added with presynthesized TiO 2 NPs (0.20 g) followed by dropwise addition of 0.1 M aqueous solution of NaOH. Subsequently, the resulting solution was stirred constantly for 5 h to achieve appropriate dispersal. Finally, the white-colored colloidal solution was refluxed for 3 h at 120°C and then allowed to cool to ambient temperature. The solution was centrifuged at 2000 rpm and washed multiple times with deionized water followed by washing with ethanol to eliminate any unreacted O 2 − ions. The sample was dried for 6 h and annealed in a muffle furnace for 3 h at 450°C, yielding TiO 2 / ZnO nanocomposite.

Synthesis of bare and La-ZnO
Hydrothermal approach was used for the synthesis of La-ZnO nanocomposite. First, different weight percentages (2-4%) of lanthanum chloride (LaCl 3 ) were dissolved in 25 mL of ethanol under constant stirring for 10 min. About, 0.473 g of Zn (CH 3 CO 2 ) 2 ·2H 2 O was added in the above solution with continuous stirring. Subsequently, 25 mL NaOH (1 mM) solution made in deionized water was added to the first solution to bring the pH of the reactants between 8 and 11. The solutions were placed in Teflon-lined (50 mL) sealed stainless steel autoclaves and placed for 6 h at 200 o C in muffle furnace. For the synthesis of bare ZnO NPs the same above procedure was used, except for the addition of La precursor. It was then allowed to cool to room temperature by its own. The resultant white solid product was centrifuged and washed with ethanol and then dried in oven at 60 o C.
Preparation of modified electrode as the WE The WE was prepared by cleaning the GCE followed by its modification with the components of modifier as discussed vide supra. Cleaning was done by polishing the surface of the electrode using 0.5-μm alumina slurry by rubbing it on a pad in a digit 8 pattern to keep the surface even. The surface was rinsed with a stream of distilled water to get rid of any unwanted particles. This process produced a clean surface with a silver mirror-like finish. The cleaned electrode was then subjected to chemical cleaning in which the electrode was dipped in a supporting electrolyte and successive voltammograms were recorded between 0 and 1.5 V at 100 mV/s. Overlaid voltammograms without any current variation pointed towards a clean surface suitable for moving on to the next step i.e., its modification.
The modified GCE was prepared via a layer-by-layer immobilization of the components of modifier (TiO 2 /ZnO and COOH-fMWCNTs) on a pre-cleaned electrode surface in a stepwise manner. To achieve this, first, a 5 µL drop of the already prepared TiO 2 /ZnO nanocomposite solution was placed on the surface of GCE using a micropipette followed by evaporation of the solvent using a dryer, then drop-casting a 5 µL drop of the COOH-fMWCNTs solution followed by drying again. The dried electrode was then gently rinsed with distilled water to get rid of the loosely bound particles of the modifier and dried again. Finally, a 5 µL drop of the analyte (MG) was applied and left to dry. This asprepared electrode designated as COOH-fMWCNTs/(TiO 2 /ZnO)/ GCE was applied for electroanalytical measurements. To test the performance of the designed nanosensor, cyclic and square wave voltammograms were recorded using 0.1 M PBS as a supporting electrolyte. A similar procedure was performed on the bare GCE to compare the results. A comparison of the analyte signal on bare and modified electrode demonstrated that GCE works better when modified, as observed by the boasted current response at COOH-fMWCNTs/(TiO 2 /ZnO)/GCE. In addition to comparison with the combination of both the modifiers and bare GCE, the results were compared with the voltammograms obtained by modification of GCE using both TiO 2 /ZnO and COOH-fMWCNTs independently.