A Potentiometric Indirect Uric Acid Sensor Based on ZnO Nanoflakes and Immobilized Uricase

In the present work zinc oxide nanoflakes (ZnO-NF) structures with a wall thickness around 50 to 100 nm were synthesized on a gold coated glass substrate using a low temperature hydrothermal method. The enzyme uricase was electrostatically immobilized in conjunction with Nafion membrane on the surface of well oriented ZnO-NFs, resulting in a sensitive, selective, stable and reproducible uric acid sensor. The electrochemical response of the ZnO-NF-based sensor vs. a Ag/AgCl reference electrode was found to be linear over a relatively wide logarithmic concentration range (500 nM to 1.5 mM). In addition, the ZnO-NF structures demonstrate vast surface area that allow high enzyme loading which results provided a higher sensitivity. The proposed ZnO-NF array-based sensor exhibited a high sensitivity of ∼66 mV/ decade in test electrolyte solutions of uric acid, with fast response time. The sensor response was unaffected by normal concentrations of common interferents such as ascorbic acid, glucose, and urea.


Introduction
Uric acid (UA) is the main end product of purine metabolism, and its excretion in urine is caused by purines that are produced in the catabolism of the dietary and endogenous nucleic acid. The production of excess uric acid may precipitate in the kidney and the lower extremities. One problem caused by the metabolism of the uric acid is gout [1]. Several epidemiological studies have suggested that the production of excess uric acid in human serum is also a risk factor for cardiovascular disease [2]. Thus, the detection of UA in human physiological fluids is necessary for the diagnosis of patients suffering from a range of disorders associated with altered purine metabolism. Recently, various uric acid biosensors have emerged from laboratories, because of the advantages of simple measurement, a short response time, high sensitivity, and high selectivity [3][4][5][6][7]. Most uric acid biosensors are based on amperometric principles [8][9][10][11]. The main problem in the practical application of many amperometric biosensors is that the electrode must be held at approximately 0.7 V [12]. The relatively high electrode potential enables other biological electroactive molecules to react on the surface of the electrode [13]. However, interferences can be reduced by preferring potentiometric configuration as described in our earlier investigations [14][15][16][17]. Recent advances in the biocompatible nanomaterials and biotechnology open a promising field toward the development of the nanostructured based electrochemical biosensing. Among the nanomaterials, zinc oxide (ZnO) is of special interest for biological sensing due to its many favorable properties like a wide direct band gap (3.37 eV) and large exciton binding energy (60 meV). In addition, ZnO has high ionic bonding (60%), and it dissolves very slowly at biological pH values.
Recently, a number of scientific investigations based on different ZnO nanostructures fabricated by various physical and chemical routes have been reported for sensing applications. These include nanowires/nanorods [18] nanotubes [19,20] combs [21,22] forks [23], fibers [24], flakes [25], composites [26], tetrapods [27], particles [28], flowers [29], sheet/disks [30], etc. Due to their unique advantages in combination with immobilized enzymes, these ZnO nanosensors offer some significant advantages owing to their small size and high surface area to volume ratios allowing larger signals, better catalysis and the more rapid movement of analyte through sensors, thus showing higher sensitivity and a lower limit of detection (LOD) as compared to those prepared from bulk ZnO devices. ZnO nanoflake (ZnO-NF) structures possess lots of interesting unique properties such as porous structures and large surface areas and there have been reports on the use of ZnO-NF structures as sensors with improved performance and higher sensitivity compared to ZnO nanorods/nanowires [25]. Moreover, ZnO has a high isoelectric point (IEP) of about 9.5, which should provide a positively charged substrate for immobilization of low IEP proteins or enzyme such as uricase (IEP ≈ 4.6) as described in our earlier investigations [31][32][33][34].
In this study, we have successfully demonstrated the potentiometric determination of uric acid with high electrochemical response by using a ZnO-NF-based sensor fabricated by a hydrothermal method. This method has many advantages such as being a low cost, simple, high yield, low temperature deposition process and also proves to be less hazardous compared to other methods. The high electrochemical response can be attributed to the unique structural properties of our sensor electrode like the high surface to volume ratios of ZnO-NFs, which can provide a favorable microenvironment for the immobilization of uricase enzyme and retain the good enzymatic activities which in turn enhances the sensitivity of sensor electrode for the analyte, as demonstrated by the detection of uric acid in the absence of a mediator.   N 4 ] that was kept in preheated an oven at 90 °C for 2-4 hours. After the growth process, the fabricated ZnO-NFs were cleaned in de-ionized water and dried at room temperature. A typical AFM image of ZnO-NPs arrays grown on the gold coated plastic electrode using this procedure are shown in Figure 1(b).
The morphological and structural studies were performed by using Scanning Electron Microscopy (SEM). The SEM images of the ZnO-NFs with as fabricated, after enzymes immobilization and after measurements are shown in Figure 2(a-c). It can be clearly seen that the wall thickness of the grown ZnO-NFs are 50-100 nm in diameter with uniform density. These ZnO-NFs were well oriented on the surface of the electrodes. The morphological and structural characteristics of the fabricated ZnO-NFs arrays can be controlled by adjusting the growth parameters.

Enzymes Immobilization on ZnO-NFs
To immobilize the uricase enzyme on the fabricated ZnO-NFs, first we have prepared an uricase solution in 10 mM PBS pH 7.4. Uricase was electrostatically immobilized by dipping the ZnO-NF-based electrode into the enzyme solution for 15 minutes at room temperature and then for drying, it was left in air for 60 min. After drying, Nafion solution (1% in methanol, 5 µL) was applied onto the electrode surface to prevent possible enzyme leakage and eliminate foreign interferences. All  When uric acid is oxidized in the presence of uricase it is turned into allantoin along with carbon dioxide and hydrogen peroxide. Due to the presence of water (H-OH), it is a high probability that allantoin will accept a proton from (H-OH) converting it to allantoinium ion, which in turn will interact with the ZnO-NFs and produce a potential change at the electrode. As the concentration of ions changes in surrounding the ZnO-NFs and the electrode potential will change [36]. The potentiometric responses of the sensor electrodes were studied in uric acid solutions made in buffer (PBS pH 7.4) with concentration ranging from 0.5 µM to 1,500 µM. During the measurements it was observed that the carbon dioxide produced does not affect the stability of ZnO-NFs as shown in SEM image of Figure 2(c) and we did not observe any substantial change in pH of the buffer solution (PBS).
The tested sensor configuration showed large dynamic ranges with an output response (emf) that was linear vs. the logarithmic concentrations of the uric acid with sensitivity around 66 mV/decade as shown in Figure 4(a). A very fast response time was noted over the whole concentration range with 95 % of the steady state voltage achieved within 8 s, as shown in Figure 4

Reproducibility, Measuring Range and Detection Limit of the ZnO-NFs Based Sensor
To evaluate the performances of the proposed sensor, we have checked the parameters like reproducibility, measuring range, detection limit, response time and selectivity, etc. The reproducibility is an important characteristic for the performance evaluation of a sensor. To evaluate reproducibility and long term stability of the proposed ZnO-NFs based sensors, we independently fabricated six sensor electrodes under the same conditions; the relative standard deviation of the fabricated sensor electrodes in standard uric acid solutions was less than 5%. The sensor to sensor reproducibility in 100 µM uric acid solution is shown in Figure 5. The mea graph as sh 0.5 µM to 1 ZnO-NFs ba sensor was curve shown more than th uric acid. It high numbe n result sh proposed ur mmobilizat n bioanalys crucial and nanoflakes e concluded th he same con electrode w Figure 1(c).  mentioned methods, we checked the selectivity and stability of the sensor by output response curve. The possible interferences present in blood that normally interfere with an amperometric uric acid biosensor include ascorbic acid (AA) urea (UR) and glucose (GL) [38]. Hence, ascorbic acid, urea and glucose were selected to affirm the selectivity of the potentiometric uric acid sensor. In the present work, upon addition of 1 mM glucose, 100 µM ascorbic acid and 1mM urea solutions in a 100 µM uric acid solution the signal changed only slightly, which indicates a good selectivity, as shown in Figure 6. This was repeated several times on new, independently prepared sensors and continued to show negligible signal response to interferences. In practical measurements, however these changes in sensor response can be neglected.

Conclusions
In conclusion, we have successfully demonstrated a simple fabrication procedure for a highly sensitive electrochemical uric acid sensor based on ZnO nano-flake-based structures. The proposed electrochemical nanosensor demonstrates immense surface area to volume ratios which provide a suitable microenvironment for enzyme loading because of its porosity that allows for very good sensitivity as compared to other ZnO nanostructures as shown in Table 1, portability and small size.  The uricase sensor retained its enzymatic activity due to strong electrostatic interaction between zinc oxide and uricase. Moreover, the developed ZnO-nanoflake-based nanosensor showed excellent performance regarding sensitivity, stability, selectivity, reproducibility and resistance to interference when the sensor was exposed to uric acid test solutions. These results revealed that electrochemical sensors based on ZnO nanoflakes have the potential to perform measurements biologically relevant to on-spot clinical diagnosis. They are also convenient to assemble into portable chip based sensing devices suitable for unskilled users.