Recently, non-enzymatic electrochemical hydrogen peroxide (H2
) sensors based on functional hybrid nanomaterials have attracted more and more attention compared to enzymatic sensors because of their advantages such as higher selectivity and sensitivity, simpler fabrication strategy, and lower price [1
]. They possess wide applications in qualitative or quantitative clinical analysis and food safety evaluation [4
]. Compared to other detection techniques such as spectrophotometry [7
], chemiluminescence [8
], and electrochemical enzymatic sensing [9
], the non-enzymatic electrochemical H2
sensors show superiority in terms of their simplicity and low cost [11
]. In particular, although the enzymatic sensors usually have unique selectivity towards a lot of biological analytes, they still possess obvious shortcomings such as susceptibility and a short service life. Previous studies have indicated that non-enzymatic electrochemical sensors could prohibit these disadvantages on one hand, and on the other hand they can obtain higher stability and better reproducibility [13
Electrospinning is a simple and efficient technique that is driven by electrical forces to fabricate pure or hybrid polymer nanofibers (PMNFs). This occurs when the electrical forces at the surface of a polymer solution or melt overcome the surface tension and cause an electrically charged jet to be ejected [16
]. By this way, we can obtain various PMNFs with adjustable length, diameter, chemical functionality, and mechanical properties by selecting various polymer precursors and changing the electrospinning parameters [19
]. For example, by incorporating nanoscale building blocks (NBBs) such as metallic nanoparticles (MNPs) or quantum dots within the spinning dope or onto the surface of PMNFs after spinning, it is possible to adjust the physical and chemical properties of electrospun PMNFs by introducing functional additives with special optical and electrical properties to create novel hybrid nanofibrous materials [20
]. In addition, the nanofibrous membrane fabricated by electrospinning has a uniform three-dimensional (3D) porous nanostructure, which consists of thousands of nanofibers with or without particular arrangement. The large surface area and high porosity of the fabricated nanofibrous PMNF membrane could provide a lot of active sites for the immobilization of analytes, affording an enhanced current response for the test molecules/biomolecules [21
]. Previously, we prepared a graphene-PMNF hybrid nanofibrous membrane by electrospinning and further utilized the created membrane materials for the fabrication of a high performance electrochemical H2
]. The obtained results indicated that it is possible to improve the electrochemical detection of analytes by incoprating other electroactive nanomaterials into the electrospun PMNFs.
Electrode materials are the bridges connecting the sensor system and current conduction system. In the biological detection aspect, oxidation-reduction reactions take place between electrode materials and biological detected objects under a given voltage, and therefore a good electrode material should possess the abilities of precise selectivity, rapid responses, low detection limit, and good electrical conductivity [21
]. Generally speaking, to guarantee both sensing and conducting properties of an electrode material, a catalyst with high selectivity and conductivity is necessary. Previously, MNPs such as Au, Ag, Pt, and Cu NPs have been thought to be potential candidates to fabricate various electrode materials for electrochemical H2
biosensors or sensors due to their good electrochemical activity toward analytes as well as their good electrical conductivity [25
]. Previous studies have indicated that the combination of NBBs with PMNFs to create hybrid nanofibrous membranes is a potential way to fabricate functional electrode materials. We have demonstrated that both one-dimensional (1D) materials like carbon nanotubes [28
] and two-dimensional (2D) materials like graphene [22
] could assist in the better dispersion of MNPs and further improve the electrical conductivity of electrospun PMNFs.
However, with the pre-processing method, the dispersion of NBBs in electrospun PMNFs is always a challenge. The huge gap in surface energy between NPs and the polymeric matrix may lead to severe aggregation and further affect the final sensing performance [30
], and the NBBs that remain trapped inside PMNFs possess a limited role for analytical detection [31
]. To solve this problem, we first fabricated amino-functionalized polyacrylonitrile (PAN) nanofibers by electrospinning a PAN and 3-aminopropyltriethoxysilane (APS) mixed precursor solution, as shown in Figure 1
A. Therefore, the as-synthesized negative-charged platinum nanoparticles (PtNPs) could be conjugated onto the positive-charged PAN/APS nanofibers through the electrostatic interaction (Figure 1
B). To fabricate an electrochemical sensor, the created PAN–PtNPs hybrid nanofibrous membrane was further utilized to modify a glassy carbon electrode (GCE), as indicated in Figure 1
C. We suggest that the fabricated electrochemical sensor could exhibit at least two advantages compared to other kinds of sensors. First, the fabricated PAN–PtNPs hybrid membrane has a 3D porous structure, which could reveal higher surface area and more active sites for analytes. In addition, the nanoscale pores in the membrane could promote the adsorption and diffusion of reactants. Second, the electrostatic assembly could mediate the ordered binding of PtNPs along PAN nanofibers with relatively high density [33
], which in some way could improve the sensitivity of the current response and enhance the sensing performance.
3. Materials and Methods
Reagents and materials. PAN (Mw = 150,000) was provided by J&K Scientific Ltd., Beijing, China. N,N-Dimethylformamide (DMF, >99.8%), sodium hydroxide (NaOH, ≥99.0%), chloroplatinic acid hydrate (H2PtCl6·6H2O, ≥99.9%), and Nafion solution were purchased from Sigma-Aldrich (St. Louis, MO, USA). Disodium hydrogen phosphate (Na2HPO4), sodium dihydrogen phosphate (NaH2PO4), ethanol, ascorbic acid (AA), uric acid (UA), and dopamine (DA) were purchased from Beijing Chemicals Co., Ltd. (Beijing, China). H2O2 (analytical grade, 30% aqueous solution) was supplied by Tianjin Dong fang Chemical Plant (Tianjin, China). The water used was purified through a Millipore system (18.2 MΩ·cm).
Electrospinning preparation of PAN/APS nanofibers. First, 1.5 g PAN was dissolved in 15 mL DMF at 80 °C and stirred for 2 h until completely dissolved to prepare the electrospinning solutions. Then, 0.18 g APS was added into the spinning solution until PAN was cooled to room temperature, followed by further stirring for 6 h. The mass ratio of APS to PAN was adjusted to 12%. The total concentration of all the solutions was 20 wt %, which is homogeneous and stable. The preparation of PAN/APS nanofibers was performed on a home-designed electrospinning apparatus produced by Beijing Technova Technology Co., Ltd (Beijing, China). For the electrospinning, 5 mL mixed solution was loaded into a 10 mL syringe (18-gauge blunt tip needle) and an electrospinning rate of 0.1–0.3 mL/h was set. In addition, an applied voltage (10 kV) and distance (about 12 cm) were utilized. The electrospun PAN/APS hybrid nanofibrous membrane was placed under ambient conditions for 48 h and then heated in a ventilated oven at 60 °C for 24 h for the subsequent conjugation with PtNPs.
Synthesis and binding of PtNPs. The platinum sol-containing PtNPs was prepared by a typical aqueous reduction method and used for binding. In brief, 25 mL NaOH (0.5 M) in glycol solution was first mixed with 25 mL H2PtCl6·6H2O (1.9 mM) under stirring. N2 was used to eliminate moisture and other organic byproducts in the solution system. Then, the mixed solution was heated to 160 °C for 3 h to synthesize the polymer-protected PtNPs. Natural cooling to room temperature occurred to obtain a dark brown, unprotected colloidal solution. The conjugation of PtNPs onto the electrospun PAN/APS mats was achieved by the in situ precipitation method. Specifically, the electrospun fiber mat was cut into rounds with a diameter of 3.0 cm and then immersed into 10 mL of platinum sol. Next, HCl (1 M) was added dropwise into the system until the pH of the system reached 4.0. After that, the mat was then washed five times with deionized water and dried thoroughly.
Preparation of PAN–PtNPs/GCE. Before the modification, GCE was polished with 1 and 0.3 μm alumina slurry, and after that, the polished GCE was further washed with ethanol and distilled water in an ultrasonic bath for 10 s, respectively. For the immobilization of nanofibrous membrane on GCE, the GCE was fixed on the drum collector, and both the drum collector and GCE were connected to the ground, and the whole process of electrospinning lasted for 2 min. Finally, the fabricated PAN–PtNPs/GCE was dried in air and stored at 4 °C for the electrochemical detection of H2O2.
Characterization techniques. SEM characterization was carried out with a JSM-6700F scanning electron microscope (JEOL, Tokyo, Japan) at 20 kV. TEM measurements were performed with a Tecnai G220 transmission electron microscope (FEI, Beijing, China) with an accelerating voltage of 200 kV. XRD (Rigaku D/max-2500VB+/PC, Shanghai, China), XPS (ThermoVG ESCALAB 250, Tokyo, Japan), and Raman spectroscopy (LabRAM HORIBA JY, Edison, NJ, USA) were used to characterize the structure of samples.
Electrochemical tests. All electrochemical tests were performed on a CHI 660A electrochemical workstation (Chenhua, Shanghai, China) at room temperature. A conventional three-electrode system was employed with a bare GCE, a Pt wire, and a KCl-saturated calomel electrode (SCE) as the working electrode, auxiliary electrode, and reference electrode, respectively. Phosphate buffer solutions (PBS, 0.1 M, pH = 7.4) that deoxygenated with highly pure N2 for 20 min were used for the electrochemical test system.