pH‑EGFET Sensor Based on the Surface Modification of MacroPSi with Au-NPs

A heterostructure of (Au-NPs/MacroPSi) was synthesized by filtering a colloidal of gold nanoparticles (Au-NPs) into MacroPSi layer using high voltage electrophoretic deposition (HVEPD). The aim of that is to enhance the performance of MacroPSi used as a pH-EGFET sensor. For this purpose, the MacroPSi with an average pore size of 1.388 μm was synthesized by anodic etching of Si wafer, and a spherical shape of Au-NPs with size varied in the range of 5 to 25 nm were synthesized in ethanol by pulsed laser ablation in liquid (PLAL) technique. The density of (Au-NPs) on the surface and inside the pores of MacroPSi was determined utilizing FESEM. The pH sensitivity, linearity, and hysteresis of the heterostructure Au-NPs/MacroPSi pH-EGFET sensor were measured in the pH range from 2 to 12. The outcomes indicated that the membrane exhibited a significantly enhanced pH sensitivity value of 83.57 mV/pH with linear regression of 99.37%, and hysteresis of 3 mV and 5 mV in the acid and base pH cycles, respectively. As well as a rapid acid and base response ˂ 5 s for a 3-units pH change was obtained. The presence of Au-NPs improve the sensitivity and stability of the sensor due to deactivate of the Si–h surface bond and enhancement the conducting on the surface of MacroPSi, and also provide better Ohmic contact to the PSi. The research work confirms the feasibility of using the hetero structures (Au-NPs/MacroPSi) as extended get for acidity and basicity detection.


Introduction
In recent times, there is an immense attention in the development of pH-sensors for biochemical and biological applications, given that numerous chemical and biochemical reactions depend on pH [1]. The first instance of a solid-state device for measuring pH across a range of ion concentrations was established in 1970 with the development of ion-sensitive field-effect transistors (ISFETs) constructed from metal oxide semiconductor field effect transistors (MOSFETs) as a substitute for the glass electrode [2]. In this class of devices, the gate requires the contact of high resistive materials like SiO 2 with a buffer solution within an electrolytic cell. The drain current is dependent on the potential accumulated at the electrolyte/ solution interface [3,4]. Nonetheless, as a result of the susceptibility of the membrane to degrade in adverse settings, the extended gate field-effect transistor (EGFET) design has been modified to extend the stability of the operation. Both platforms have essentially the same theoretical framework [5,6]. The difference is that the EGFET has an added appendage (sensing gate) consisting of a low impedance material such as TiO 2 , RuO 2 , and V 2 O 5 , which is utilized to expose the sensor directly to the electrolyte solution, dissociate the FET from the electrolyte, and shift the electric potential to the gate terminal of a commercial MOSFET [3]. This approach provides long-term stability by protecting the FET from any possible chemical damage caused by the penetration of ion species in solution. In addition, this structure allows swapping the sensing membrane instead of fabricating a whole new device. This leads to several other benefits that include low cost, simple packaging, and non-responsiveness to both light and temperature [7,8].
Porous silicon (PSi) is a type of silicon with nanopores etched into its microstructure, thereby increasing its surface to volume ratio in the order of 700 m 2 /cm 3 [9]. PSi is produced by chemically or electrochemically etching monocrystalline silicon in a solution of hydrogen fluoride (HF). Depending on the electrochemical conditions, PSi can exhibit pores and crystallites of nanometric size [10]. A large surface area interface composed of a variety of pores can make PSi one of the most important porous materials having a broad variety of applications. In addition, the pores allow the penetration of chemical and biological substances to change the optical and electric behavior of the original system [11,12].
A number of studies have sought out evidence and created remarkable concepts supporting the feasibility of PSi and Si nanostructures-based pH-EGFET devices [13,14]. The obtained sensitivities for different PSi morphologies exceed the Nernst theoretical limit of about 59.2 mV/pH, with relatively pure hysteresis and unreliability, mostly due to the unstable nature of the material. This is attributable to the volatile nature of its native surface/interface with a metastable Si-Hx ligand [15]. The metastable hydro-silicon can be subjected to impulsive oxidation at room temperature conditions, leading to the breakdown and formation of structural defects at the surface [16]. The issue of poor electrical contact on PSi also arises, resulting in low electric conductivity of the PSi. Thus, the surface needs to be passivated to prevent degradation and ensure the fabrication of a stable PSi-based device. The replacement of the surface hydrogen with alternative chemical species seems to be desirable to achieve this aim [16][17][18]. The passivation of the surface via chemical treatment/doping with noble metal ions is also a potentially viable option. Metals such as Ag, Cu, In, and so on, have been utilized to alter the surface of porous silicon to improve its stability for various applications [19][20][21]. The sensing membrane of the EGFET electrode must be highly conductive and capable of transmitting sensing signals without difficulty [22]. Thus far, several techniques have been employed to enhance the conductivity of sensing membrane, which include doping with a variety of elements or surface decoration of nanostructures with noble metal particles [23,24]. In recent times, the sensitivity of the pH-EGFET sensor has been significantly enhanced by surface decoration of N-MacroPSi with palladium nanoparticles (Pd-NPs) [25].
A colloidal solution of metallic nanoparticles can be made by means of pulsed laser ablation in liquid (PLAL) technique. In PLAL, laser pulses are applied to a solid metallic target submerged in a liquid [26]. A colloidal nanoparticle can be deposited with various techniques. For example, the electrophoretic deposition method (EPD) applies a homogeneous external electric field to move the suspended particles from the solution toward the surface of the substrate [27,28]. The thickness of the deposited film of NPs is dependent on a range of factors that include particles concentration of a solution, deposition duration, and applied electric field. Interestingly, it is worth pointing here to the novelty of this work, which is one of the most noticeable facts about the present work. The novelty of the work can be defined by the Au-NPs/MacroPSi EGFET based pH sensor, which was investigated for the first time.
To enhance the performance of MacroPSi used as a pH-EGFET sensor, a hetero structure of Au-NPs/MacroPSi was synthesized by filtering a colloidal of gold nanoparticles (Au-NPs) into the MacroPSi layer. Herein, the dense Au-NPs growth provides a passivation layer to prevent degradation of surface atoms and improve the conductivity of MacroPSi. In this way, the presence of Au-NPs improves the sensitivity and stability of the MacroPSi EGFET pHsensor. It is well known that the sensitivity of the MacroPSi EGFET pH-sensor is dependent on the surface morphology and sensing conductivity of the MacroPSi.
Based on the output of our previous study (MacroPSi synthesized by anodic etching of a p-type (100) monocrystalline Si wafer for use as a pH-EGFET sensor), the surface of MacroPSi (prepared at the same condition) was modified with Au-NPs. Au-NPs were synthesized on the surface and filtered inside MacroPSi using high voltage electrophoretic deposition (HVEPD) method.

Synthesis of Metallic Gold Nanoparticles (Au-NPs)
A colloidal solution of (Au-NPs) was synthesized using the PLAL technique. The PLAL process is illustrated in Fig. 1a. Au target (0.5 mm thickness, purity of 99%) was first immersed in 10 ml of ethanol. Afterwards, Q-switched Nd:Yag laser beam with 1000 mJ was applied on the Au target. The Au target was subjected to 100 pulses of the laser beam, followed by a pause of 30 s to allow the laser head to cool down, after which the hit of 100 pulses was resumed. This process was repeated 15 times to achieve 1500 pulses on the Au target. Using the Beer-Lambert Law, the concentration of Au-NPs in the solution was determined to be 5.11 nM based on the absorbance spectra of the collide solution of Au-NPs, as explained in the (Supplementary Material Appendix 1). The Au-NPs were found in the ethanol beaker with a purplish red color. These Au-NPs were used in this work to fill the pores of the MacroPSi thin film.

Substrate Fabrication (MacroPSi)
MacroPSi was produced by anodic etching of monocrystalline p-type Si wafers (polishing corporation of America, RES: 1-20Ω, Orient: 100). The anodization process is illustrated in Fig. 1b. The anodic cell consists of a Teflon beaker with a round opening (diameter of 1.5 cm 2 ) at the bottom. The base of the beaker under the Si substrate symbolizes the anode of the cell. This component consists of an inscribed Cu base that is linked to the positive terminal of the DC power adapter. The cathode is represented by the upper portion of the beaker, which consists of a 1.5 cm 2 platinum plate linked to the negative terminal of the same power supply. A standard process based on the RCA method was used to clean the Si wafer. Before performing the electrochemical etching, the sample was immersed in a solution of hydrofluoric acid (HF: 49%) and de-ionized (DI) water (mixed at 1:10) for 60 s to remove the oxide layer that is naturally produced. Subsequently, the Si wafer was positioned at the base of the Teflon beaker attached to the Cu base so that the needed etching current may flow from the bottom to the top of the polished sample surface. A rubber O-ring was inserted between the top portion of the Si wafer and the Teflon baker to avoid the leaking out of solution. The anodization process was carried out in a combined solution of HF acid (49%), and ethanol (99.5%) mixed at a volume ratio of 1:4, while the wafer was anodically polarized at a steady current density of 10 mA/cm 2 for one hour. Following the anodization procedure, the electrolyte was replaced severally with pure ethanol to remove residual HF from the pores, and the resulted MacroPSi was kept in the cavity for deposition of Au-NPs.

Synthesis of Hetero Structures (Au-NPs/MacroPSi)
High voltage electrophoretic deposition (HVEPD) was employed for the infiltration of Au-NPs onto the MacroPSi. The same anodic cell that is used for fabricating MacroPSi is also used for preparing heterostructures (Au-NPs/ MacroPSi), as illustrated in Fig. 1c. The anodic cell with freshly etched porous silicon fixed in it, and with replacing a DC power supply with a high voltage one (the cathode connected to PSi and the anode connected to Pt electrode) and the electrolyte replaced by a suspension solution from fresh un-aggregated (Au-NPs + ethanol) that was prepared by (PLAL). The applied potential increased slowly from (0 to 2) KV for 30 s. Finally, the samples were washed with ethanol and aerated under nitrogen flow. Preparation of the Ohmic contact of the sensing gate to commercial MOSFET involved deposition of Au onto the exterior area/surface of the MacroPSi through a mask by means of the automatic thermal evaporation technique (Edwards-306).

Characterization Techniques
X-ray diffraction (XRD) (Burker Advance-D8) and transmission electron microscopy (TEM) (Zeiss Libra-120) were employed to characterize the crystal structure and particle size of colloidal Au-NPs in ethanol, respectively. Field emission scanning electron microscopy (FESEM) (Nova Nano SEM-450) attached with energy dispersive X-ray spectroscopy (EDX) was utilized to investigate the morphological features and elemental composition of the samples. The program Scanning Probe Image Processing (SPIP) was used to examine the resulting TEM and FESEM images. Atomic force microscopy (AFM) (Dimension edge-Bruker) was used to quantify the surface roughness and to determine the surface area of MacroPsi and AuNP/MacroPSi. The setup shown in Fig. 2a was utilized to evaluate the sensing properties of the fabricated Au-NPs/MacroPSi pH-EGFET sensor. For a variety of pH buffer solutions (2, 4, 6, 8, 10, and 12), the I-V curves of the pH-EGFET sensor were determined. The values were then used for the membrane's linearity and sensitivity calculations as follows: The EGFET structure comprises a sensing membrane (Au-NPs/ MacroPSi) with a 1.5 cm 2 area, positioned within a cavity that has a design similar to that of the electrochemical etching system (Fig. 1b). The Au electrode of the sensing gate was connected to the gate terminal of the standard N-MOSFET (HEF4007UBD) using a Cu ring. Two source One of the source meters (Drain) was used to apply voltage between drain and source (V DS ), while the other (Gate) was utilized to apply the reference electrode voltage (V ref ) to the sensing membrane (Au-NPs/MacroPSi) by using the (Ag/ AgCl) reference electrode that was submerged in the buffer solutions within the cavity. The setup depicted in Fig. 2b was utilized to calculate the hysteresis and response time of the sensing membrane by monitoring the potential difference between the sensing membrane and the reference electrode over time. In order to acquire the voltage data, one of the Keithley devices was transformed into a voltmeter and afterwards connected directly between the sensor membrane and reference electrode, eliminating the commercial MOSFET. For hysteresis measurements, the production of buffer solutions followed a pH (7 -4 -7 -10 -7) cycle. The data for each solution was collected at a rate of 5 min. In order to measure the response time, the pH value is drastically altered from acidic to basic, starting with a buffer solution pH7. A controlled amount of HCL or KOH is added to alter pH7 to pH4 or pH10, respectively.

Structural and Morphological Analyses
The synthesized Au-NPs were subjected to structural and morphological analyses using XRD and TEM, respectively. Figure 3 presents the XRD pattern, particle size distribution, and TEM micrographs of the Au-NPs. The XRD spectrum of Au-NPs displayed in Fig. 3a depicts a number of diffraction peaks located at different 2θ angles that are characteristic of Au, which match well with those of its standard pattern (ICSD (Au)-: 01-089-3697). The main distinctive spectral peaks located at 2θ = 38.17, 44.39, 64.64, and 77.65 correspond to standard (111), (200), (220), and (311) Bragg reflections of face center cubic structure, respectively. The high intensity peak at 2θ = 38.17 implies that zero valent gold preferentially grows in the (111) direction. This refers to the production of molecular-sized compounds having a recurrent 3D pattern of atoms or molecules separated by an analogous distance. This XRD spectrum is typical of pure Au-NPs [29]. The TEM analysis of the spherically shaped Au-NPs (Figs. 3b and c) shows that their particle size distribution varies from 5 to 25 nm. Additionally, the TEM images illustrate the non-aggregation or random dispersion of the Au-NPs.
Furthermore, FESEM was used to evaluate surface morphological variations, layer thickness of the MacroPSi, and the density of Au-NPs growth on the top and within the pores of MacroPSi substrate. The top view image of the MacroPSi illustrated in Fig. 4a depicts densely distributed holes with a variety of pore sizes that are almost circular in shape. The average pore diameter and porosity were determined to be 1.388 μm and 36.3%, respectively, which were calculated using the SPIP software, as displayed in Fig. 4a. The cross section FESEM image at the right of Fig. 4a shows discrete cylindrical-shaped pores with smooth walls penetrating about 12 μm deep into the bulk silicon substrate. However, Fig. 4b presents enlarged top view and cross-section FESEM images of Au-NPs deposited on the surface and within the MacroPSi layer. The FESEM images show the accretion of Au-NPs on the surface of MacroPSi without complete pore closure. The Au-NPs aggregated into clusters with an average diameter of 26.72 nm, which was measured using the SPIP software, as seen in Fig. 4b. The cross-section images reveal the presence of Au-NPS inside the pores. Figure 5 displays 3D AFM images of synthesized MacroPsi and AuNP/MacroPsi for an area of 10 µm 2 . Using (Nano scope analysis) software, the output AFM image was analyzed to determine the percentage of surface area difference and several roughness parameters, including the EDX was used to explore the modification in the surface composition of the MacroPSi substrate due to the depositing of Au metallic nanoparticles. The EDX spectrum of

pH Sensing Properties
The pH-sensing process of adsorbed surface ions in ISFET and EGFET is comparable. By surface interactions, the charging process of oxide is described by site-binding and double-layer approaches. The surface potential is determined by protonation or deprotonation reactions that take place at the membrane surface, resulting in a net charge which is either positive (H 2+ ) for low pH or negative (O − ) for high pH [7,13]. Consequently, the charge of the surface potential voltage at the contact between the Au-NPs/MacroPSi and electrolyte controls the membrane's pH sensitivity, which can be computed using the following expression [30,31]: where pH pzc ,q,K , T and represent the pH value at the level of neutral charge, elementary charge, constant of Boltzmann, absolute temperature, and sensitive parameter, respectively. can be calculated using the underlying equation [32]: where N s and C DL denote the surface site density and electrical double layer capacitance, respectively, while K a and K b signify the constant values for acidity and basicity, respectively.

KTC DL
The surface potential voltage ( o ) produces an electric field at the interface between the sensitive membrane and the electrolyte interface, thereby altering the resistance of the MOSFET channel and modifying the drain-source current [14,33]. In the next steps, after successful adsorption and infilling of Au-NP S on the surface and within the pores of MacroPSi, respectively, the samples were fabricated into pH-EGFET sensors. The sensitive properties of the Au-NPs/ MacroPSi membrane are studied in the linear and saturation regimes as shown:   Fig. 7b. The pH voltage sensitivity and linearity may then be determined as outlined below [13]: The pH voltage sensitivity for Au-NPs/MacroPSi EGFET was estimated to be 83.57 mV/pH with a linearity value of 99.37%,

B-Saturation region (Sensitivity and linearity)
As a channel is formed, saturation arises, which enables the interchange of current flow between the source and drain. The I DS varies with dissimilar concentrations of hydrogen ions amassed on the surface in response to the buffer solution's pH. The current sensitivity and linearity are obtainable from the I DS -V DS curves for different buffer solutions. For the saturation portion of EGFET, the relationship between I DS and V DS is given by the equation below [31]: where m signifies the channel length modulation factor. The I DS -V DS output curves in the saturation portion are Fig. 8a for a pH range of 2 to 12 at V ref of 3 V. A downward shift in the I DS -V DS curve is discernible as pH increases. The low pH indicates a high buildup of H + ions on the surface of the membrane that induces the transfer of additional positive voltage applied to the gate, thus extending the dimensions of the conducting channel but decreasing the depletion layer, leading to a rise in current flow. Contrarily, for high pH solutions, where the high pH signifies low and high accumulation of H + and OH − ions, respectively, corresponding to the introduction of a negative gate voltage decreases the conducting channel but increases the depletion layer, leading to a decline in current flow. Further, this distinction between acid and basic responses can be seen in Fig. 8a. In fact, the fast ion exchange of tiny mobile H + ions, which predominate in acidic solutions, is responsible for the quick reaction in acidic solutions. Numerous investigations have proven that the sensor reacts much better in acidic solutions compared to basic solutions [34]. The sensor sensitivity was determined from the plotted graph of I DS vs pH at V DS = 3 V, as shown in Fig. 8b. The current sensitivity to pH obtainable from the saturation regime is calculated using Eq. 7 below [13]: For the saturation region, the calculated pH current sensitivity value for the Au-NPs/MacroPSi EGFET sensor was equal to (-2.34 µA 0.5 /pH), with a linearity of (99.95%). The results exhibit that both V ref and I DS show a strong linear dependence on pH in the linear and saturation regions, respectively. This study also evaluated the reproducibility of Au-NPs/MacrPSi. Reproducibility is the ability of a sensor to be reconfigured in order to obtain identical experimental results. A biosensor is regarded as precise when it produces identical findings each time a sample is measured, and it is deemed accurate when it provides a number that is close to the mean of several measurements of the sample [35]. Using the same method and conditions, the voltage sensitivity of three samples were found to be 83.57, 81.17, and 82.43 mV/ pH, with an average value of 82.39 mV/pH and a standard deviation of (SD ± 1.2). However, the present values for current sensitivity are found to be -2.34, -2.27, and -2.29 µA 0.5 /pH, with a standard deviation of (SD ± 0.03) and a mean value of 2.3 µA 0.5 /pH. The (I-V) curves for assessing the sensitivity in both the linear and saturation regions are shown in (Supplementary Material Appendix 2). Au-NPs/MacrPSi exhibited a high degree of reliability, as evidenced by the outcome result.

Hysteresis and Response Time
The hysteresis is called the memory effect, and it is anchored in the chemical reactions between ionic species in the buffer solution and slow-reacting surface sites underneath the membrane's surface as well as the defects of the membrane's surface [31]. The structural defects on the surface or underneath the Au-NPs/MacroPSi membrane were determined using hysteresis measurements. These measurements are very important since the intrinsic defects can affect the sensitivity of the Au-NPs/MacroPSi pH-EGFET sensor. Delay in pH response is denoted by high value hysteresis, which is indicative of the presence of surface defects in the membrane [36]. The system shown in Fig. 2b was utilized to obtain the sensor gate's hysteresis, which is derived from the voltage differential between the sensing gate and reference electrode. The output voltage is measured in the absence of MOSFET, which can be estimated using the equation below [3]: where V + in and V − in denote the voltages at the Keithley's two input terminals, whereas V ref and V sensing−gate denote the voltages at the reference electrode and the sensing gate, respectively. Figure 9 shows the hysteresis effect, where the output voltage is measured as a time-dependent for an assortment of buffer solutions in a cycle of (pH7 → pH4 → pH7 → pH10 → pH7), with each cycle having a duration of 5 min. Afterword, hysteresis was determined as a difference between the average output voltage of two consecutive pH 7, as depicted in Fig. 9. The net hysteresis value for the acid loop (pH7 → pH4 → pH7) was determined to be 3 mV, while the value for the base loop (pH7 → pH10 → pH7) was raised to 5 mV. A solid state potentiometric pH sensor's response time is known as the time required for its electromotive force to reach 90% of its equilibrium value after immersing the sensor in a solution of a given pH value [37]. In order to determine the sensor's response time, the pH value is significantly altered from acidic to basic (or vice versa), and the output potential is monitored as a function of time. Figure 10 depicts membrane sensing (V out -Time) curves for a three-units pH change in acidic and basic solutions. The response time in acid was found to be 3.22 s when the buffer solutions decreased from pH7 to pH4, whereas the reaction time in base was 4.88 s when the buffer solutions rose from pH7 to pH10. In the acidic zone, the majority of sensors exhibit a shorter reaction time compared to alkaline solutions [38]. The discrepancy in hysteresis and response time values between the acid and base loops is owing to the different diffusion rates of H + and OH − ions at surface sites beneath the membrane. H + and OH − ions can be absorbed and separated from the sensor surface. Due to their diminutive size, H + ions may quickly travel through the solution and penetrate the sensor surface. In contrast, OH − ions cannot travel as readily as H + ions due to their significantly bigger size. The different hysteresis and response time in the acid and the base loop is shown in Figs. 9 and 10. Table 1 shows the low value of calculated hysteresis in comparison to values reported in previous investigations. Furthermore, variation in the output voltages of pH 10 and pH 4 is discernible from Fig. 9. According to Eq. 8, where pH 10 is denoted by a large quantity of OH − ions and negative voltage, the sensing membrane displays a sluggish or slow reaction to the electrolyte ions due to the limitation of H + ions and the membrane's limited ability to exchange metal ions for H + ions. This occurrence will produce a negligible value of V sensing-gate due to the minor charge on the surface of the membrane, resulting in a significant difference between V ref and V sensing-gate , thereby generating a high output voltage. On the contrary, in the case of pH 4, with its relatively large quantity of H + and positive voltage characteristics, the membrane displays a fast response to electrolyte ions, leading to high V sensing-gate . This event results in a relatively smaller difference between V ref and V sensing-gate compared to the basic solutions. Accordingly, low output voltages were acquired for low pH solutions, as illustrated in Fig. 9.
In addition to the hysteresis, the measured current-voltage sensitivity of Au-NPs/MacroPSi pH-EGFET sensor was also evaluated against earlier data for different PSi structures (including the result of bare MacroPSi utilized as pH-EGFET and published in reference [40]), as summarized in Table 1. The good sensitivity and low hysteresis values of the Au-NPs/MacroPSi pH-EGFET sensor could be Fig. 9 Hysteresis characteristics of prepared Au-NPs/MacroPSi EGFET pH sensor Fig. 10 The response time curves for (a) acid (pH7 to pH4), and (b) alkaline (pH7 to pH10) of Au-NPs/MacroPSi membrane attributed to the presence of Au-NPs on the top and inside the pore of MacroPSi. This phenomenon is explained as follows: the Au-NPs work as a passivation layer to prevent the degradation of surface atoms. The surface of PSi is unstable because of Si-Hx metastable ligands, which can go through spontaneous oxidation to generate a non-conductive oxide layer [41]. Therefore, the colloidal Au-NPs substitute the hydrogenated surface of PSi to improve the stability of the MacroPSi pH-EGFET sensing gate, which is an important characteristic for pH-EGFET performance.
Secondly, it is proven that the oxidation of PSi occurs during the deposition of colloidal Au-NPs due to the creation of Au islands on PSi surface, which increases the formation of a thin SiO 2 layer between the aggregates of Au-NP. Hence, the deposition of colloidal Au-NPs on PSi promotes the oxidation of substrate [20]. The presence of a thin oxide layer is critical because surface reactions and the number of binding sites (Si-O) on the membrane surface control pH sensor performance. The oxide layer functions as a free acceptor on the surface of the sensor to attract hydrogen ions. As a result, the surface potential voltage (Ψ) of the AuNPs/MacroPSi pH-EGFET sensor increases concurrently with the number of these sites ( N s ). These processes enhance the pH sensitivity and linearity.
Furthermore, the thinner of oxide layer and dense metallic Au-NPs on the surface and deeper inside the pore are very important to improve the conductivity and provide better ohmic contact to the sensing gate. The Au-NPs improve the conductivity by providing additional amounts of carrier charges on the sensing gate surface, which then transmit sensing signals to the gate terminal of standard MOSFET without difficulty, subsequently increasing I DS .
Finally, it is noted that the voltage sensitivity of the Au-NPs/MacroPSi pH-EGFET sensor is relatively higher than the optimal Nernst limit of 59 mV/pH. For site binding calculations, the maximum predicted sensitivity from the Nernst equation or site binding concept is generated from a flat surface, while the high surface to volume ratio supplied by the Au-NPs combination with MacroPSi, providing a large number of binding sites on the surface, would concurrently raise the membrane surface's potential voltage, hence boosting sensitivity to pH.
Furthermore, the sensor's sensitivity is determined by the size of the buffer ions and the decrease in potential across the diffusion layer. A large amount of counter-ions are needed to increase the pH sensitivity over the Nernst limit. As a certain size of the counter ions is exceeded, the crowding effect of the ions accumulated at the surface of the sensor, coupled with the buffering state of the electrolyte, results in elevated H + activity [42]. A number of research studies similarly observed pH sensitivity values that are greater than the optimal Nernst limit, as shown in Table 1.

Conclusion
The research highlights the performance of Au-NPs deposited on MacroPSi using the HVEPD technique, with the aim of enhancing the stability and sensitivity of the MacroPSi EGFET pH-sensor. The particle size of the aggregation of Au-NPs at the surface and deeper inside 12 µm of MacroPSi ranged from 5 to 25 nm. The pH-sensor exhibited super-Nernstian sensitivity and linearity of 83.57 mV/pH and 99.37%, respectively, across the pH range of 2-12. Furthermore, the sensor exhibited a 5 mV hysteresis that was relatively low. It can be inferred that the sensitivity of the MacroPSi pH-EGFET sensor is dependent on the surface morphology and sensing gate conductivity of the MacroPSi. The dense Au-NPs growth provides a passivation layer to prevent degradation of surface atoms, improves the conductivity of MacroPSi by forming a thinner oxide layer on the surface, and increases the carrier charge near the surface for enhancement of the easy movement of the electron due to high mobility and low resistivity, which is required for the sensing gate of EGFET. The findings suggest that the Au-NPs/MacroPSi pH-EGFET sensor is a viable alternative for pH measurement in various liquids.