Portable visual and electrochemical detection of hydrogen peroxide release from living cells based on dual-functional Pt-Ni hydrogels

It is important to monitor the intra-/extracellular concentration of hydrogen peroxide (H2O2) in biological processes. However, miniaturized devices that enable portable and accurate H2O2 measurement are still in their infancy because of the difficulty of developing facile sensing strategies and highly integrated sensing devices. In this work, portable H2O2 sensors based on Pt-Ni hydrogels with excellent peroxidase-like and electrocatalytic activities are demonstrated. Thus, simple and sensitive H2O2 sensing is achieved through both colorimetric and electrochemical strategies. The as-fabricated H2O2 sensing chips exhibit favorable performance, with low detection limits (0.030 μM & 0.15 μM), wide linearity ranges (0.10 μM–10.0 mM & 0.50 μM–5.0 mM), outstanding long-term stability (up to 60 days), and excellent selectivity. With the aid of an M5stack development board, portable visual and electrochemical H2O2 sensors are successfully constructed without complicated and expensive equipment or professional operators. When applied to the detection of H2O2 released from HeLa cells, the results obtained by the developed sensors are in good agreement with those from an ultraviolet‒visible spectrophotometer (UV‒vis) (1.97 μM vs. 2.08 μM) and electrochemical station (1.77 μM vs. 1.84 μM).


Introduction
H 2 O 2 is one of the most important metabolic products and plays a key role in the proliferation, differentiation, and migration of cells under physiological conditions 1,2 .However, excessive levels of H 2 O 2 may be associated with various pathological conditions, such as cancer, Alzheimer's disease and Parkinson's disease [3][4][5] , which makes the accurate measurement of H 2 O 2 urgent and important for preventing, diagnosing and treating these diseases.Among numerous analytical strategies for H 2 O 2 determination, colorimetric and electrochemical approaches are considered the most powerful and versatile approaches due to their advantages of high sensitivity, simple operation and low cost [6][7][8] .Nevertheless, highly miniaturized and cost-effective H 2 O 2 sensors that do not rely on large equipment and professional operators have rarely been explored.This is mainly due to the difficulties of developing facile sensing strategies and the efficient integration of both sensing chips and signal processing into sensing platforms 9 .In addition, typical H 2 O 2 sensors are generally based on natural enzymes (e.g., horseradish peroxidase (HRP)), which are relatively fragile and expensive 10,11 , thus hindering their widespread application.Therefore, it is highly desirable to develop sensing materials with excellent catalytic activities to bypass the limitations imposed by natural enzymes and to fabricate H 2 O 2 sensing devices that can meet the need for immediate, portable, and sensitive H 2 O 2 detection.
Nanomaterials with properties such as mimicking enzymatic activities or electrocatalytic functions have attracted growing interest in recent years [12][13][14] .With the accelerated development of material science and technology, many nanozymes or electrocatalytic nanomaterials to replace natural enzymes owing to the easier largescale production, lower cost, more tunable catalytic activities and higher stability of the nanomaterials [15][16][17] .Metal aero-/hydrogels derived from nanoparticles are an emerging type of self-supported, three-dimensional (3D) porous nanomaterials with excellent catalytic activity.They have gained great attention and displayed unprecedented potential in the research areas of electrocatalysis, nanozymes and biosensing [18][19][20] .For instance, Zhu et al. proposed a dopamine-induced Au hydrogel that exhibited high glucose oxidase-like and peroxidase-like activities, offering a new approach to designing biosensors with metal gel-based nanozymes 21 .The fascinating porous structures and large surface areas of the metal gels can greatly improve the diffusion of the substrate and provide a large number of active sites, which thus substantially enhances the sensitivity [22][23][24] .The 3D interconnected networks of the metallic skeleton guarantee high numbers of electron transfer pathways for catalytic reactions 25 .In addition, improved retention of the catalytic activity can significantly increase the long-term stability of biosensing compared to that of natural enzymes 26 .Therefore, metal gel sensing materials with these outstanding features will open up whole new vistas for the construction of sensing devices 27,28

Microstructures of the Pt-Ni hydrogels
The Pt-Ni hydrogels were prepared via a fast and simple coreduction of a mixed metal salt solution by sodium borohydride (NaBH 4 ).Scanning and transmission electron microscopy (SEM and TEM) were first carried out to evaluate the morphology and microstructure of the representative PtNi 3 hydrogel (see Fig. 1a-d).It displayed a highly porous dual gel structure composed of interfused nanowire networks and crumpled nanosheets (Fig. 1a, b), which provided a large specific surface area and ensured high sensitivity for biosensing.The interplanar spacing value measured from the nanowire was 0.211 nm in the high-resolution TEM image (Fig. 1c), which could be assigned to the (111) facet of Pt.This value was smaller than that of metallic Pt, indicating the possible formation of a Pt-Ni alloy.Moreover, the interplanar spacing of the nanosheets was 0.261 nm, indexing to the (100) facets of Ni(OH) 2 .The diffraction peaks at 41.83°, 71.16°, and 84.26°were located between those expected for metallic Pt (PDF 04-0802) and Ni (PDF 04-0850), further demonstrating the lattice contraction of Pt and the formation of a Pt-Ni alloy in the PtNi 3 hydrogel (Fig. 1e).Moreover, the diffraction peaks at 33.48°and 59.70°corresponded to the (100) and (003) reflection planes of Ni(OH) 2 , respectively.Both the TEM and X-ray diffraction (XRD) results revealed the dual-structure gels of the Pt-Ni alloyed nanowires and Ni(OH) 2 nanosheets.In addition, the high-resolution X-ray photoelectron spectroscopy (XPS) patterns of Pt 4 f and Ni 2p showed metallic and oxidative states, respectively, with electron transfer from Ni to Pt (Fig. 1f-h).
Pt-Ni hydrogels with different Pt/Ni atomic ratios (e.g., PtNi and PtNi 5 hydrogels) could be obtained using this standard synthetic approach by tuning the amounts of metal precursors and NaBH 4 (see details in the Experimental Section and Fig. S1).Additionally, pure Pt and Ni hydrogels were synthesized for comparison by reducing only the Pt or Ni precursor.From the SEM and TEM images of these hydrogels, wrinkled nanosheets were almost invisible in the pure Pt and PtNi hydrogels and clearly observed in the PtNi 5 hydrogel, indicating that the amount of Ni(OH) 2 increased as the proportion of Ni increased.
Peroxidase-like activity and H 2 O 2 colorimetric sensing H 2 O 2 is an important biomarker in living cells and an intermediate product in the oxidation of many biomolecules (e.g., glucose, lactate and alcohol), thus making H 2 O 2 measurement a prevalent application in analytics and diagnostics [29][30][31] .It is well known that Pt-based nanomaterials show peroxidase-like activity and have great potential in colorimetric H 2 O 2 sensing 32 .First, the peroxidase-like characteristics of the PtNi 3 hydrogel were investigated through a 3,3,5,5-tetramethylbenzidine (TMB)-induced chromogenic reaction and UV-vis absorption spectra.As displayed in Fig. 2a, neither the PtNi 3 hydrogel nor the H 2 O 2 solution and TMB yielded a color change or obvious absorption peak.In contrast, the mixed solution of TMB and PtNi 3 hydrogel turned from transparent to blue in the presence of H 2 O 2 .The timedependent absorption spectrum in Fig. S2 shows that the curve reached a steady state within 3 min, indicating a rapid response time.The characteristic peak at 652.0 nm was assigned to the oxidation of TMB (ox-TMB), demonstrating the peroxidase-like activity of the PtNi 3 hydrogel.In addition, a terephthalic acid (TA)-induced chromogenic reaction was carried out to explore the A characteristic peak at 430 nm appeared only when there were both H 2 O 2 and PtNi 3 hydrogels in the solution (Fig. 2b).This phenomenon was due to the fluorescent product of the reaction of TA with hydroxyl radical (•OH) 33 .Therefore, the nature of the catalytic behavior of the PtNi 3 hydrogel can be attributed to the generation of •OH 34 .
For comparison, we tested the peroxidase-like activity of the Pt, Ni, PtNi and PtNi 5 hydrogels.As seen in Fig. 2c, d, all the Pt-based hydrogels exhibited significant absorbances (Pt: 0.062, PtNi: 0.068, PtNi 3 : 0.117, PtNi 5 : 0.063) in the presence of 0.20 mM H 2 O 2 , while only a slight signal was observed in the pure Ni hydrogel.The steady-state kinetic assay of the PtNi 3 hydrogel was performed by varying the TMB or H 2 O 2 concentrations under the same conditions.Control experiments were also carried out for the PtNi, PtNi 5 , pure Pt and Ni hydrogels.Both TMB and H 2 O 2 followed the standard Michaelis-Menten model well for these hydrogels (Figs.S3-S7).The corresponding Michaelis constant (K m ), maximum initial velocity (V max ) and catalytic constant (K cat ) of the PtNi x , pure Pt and Ni hydrogels were calculated according to the Lineweaver-Burk curves and are shown in Table S1 and Fig. 2e, f 34,35 .The K m values of the Pt-based hydrogels for both H 2 O 2 and TMB were much lower than that of HRP, suggesting their higher affinity to these two substrates 36 .The higher K cat of these hydrogels with respect to HRP indicated higher catalytic activity per unit concentration 37 .Most importantly, the PtNi 3 hydrogel displayed the highest affinity and catalytic activity (K m (TMB) : 0.031 mM, K m ( H 2 O 2 ) : 0.67mM, V max (TMB) : 6.08 × 10 −8 M s −1 , V max (H2O2) : 4.52 × 10 −8 M s −1 ) compared to the natural and artificial enzymes (Table S1).This can be ascribed to the following three factors.(1) The highly porous structures and large surface area of the PtNi 3 hydrogel provide excellent mass transfer and rapid electron transfer between the hydrogels and the substrates 25 .(2) Considering the low catalytic activity of the Ni hydrogel, we can infer that the high peroxidase-like activity of the PtNi 3 hydrogel was promoted mainly due to the synergistic effect between Pt and Ni in the alloyed Pt-Ni nanowires 38 .(3) Moreover, the generated Ni(OH) 2 nanosheets can significantly enhance the affinity of the PtNi 3 hydrogel to the substrates by increasing the specific surface area and accelerating the desorption of the •OH adsorbed on the PtNi alloy, thus improving its catalytic performance [39][40][41] .However, excessive Ni(OH) 2 on the surface of the hydrogel may restrict the contact between the substrates and the alloyed Pt-Ni nanowires, reducing the catalytic performance (e.g., PtNi 5 hydrogel) and resulting in a volcano-type trend with increasing Ni content, as shown in Fig. 2d.
Taking into account the excellent peroxidase-like property, the PtNi 3 hydrogel shows great potential for constructing a colorimetric H 2 O 2 sensor.The working temperature and pH value were optimized to maximize the sensing performance.
Similar to the natural enzymes, the catalytic activity of the PtNi 3 hydrogel exhibited temperature-dependent (Fig. S8A,  B) and pH-dependent (Fig. S8C, D) behavior.Therefore, the optimal conditions of 25 °C and pH = 5 were selected for the subsequent sensing experiments.
We then systematically evaluated the analytical performances in H 2 O 2 colorimetric sensing based on the PtNi 3 hydrogel.Figure 3a illustrates the UV-vis spectra and corresponding optical images of the TMB solution with different concentrations of H 2 O 2 .As shown in Fig. 3a-c, the absorbance of ox-TMB at 652.0 nm increased with increasing H 2 O 2 concentration from 0.10 μM to 10.0 mM and revealed a wide linearity range.The LOD was calculated to be 0.030 μM (S/N = 3).Furthermore, the tiny relative standard deviation (RSD) of nine repeated measurements (less than 1.50%) indicated high repeatability of the H 2 O 2 measurement.In addition, the selectivity of the PtNi 3 hydrogel-based colorimetric sensing was evaluated.The results in Fig. 3d show almost no significant signal changes in the absorbance with respect to 0.10 mM H 2 O 2 after the addition of 0.20 mM uric acid (UA) (3.24%), 0.20 mM ascorbic acid (AA) (3.06%), 0.20 mM glucose (5.38%), 0.20 mM potassium chloride (KCl) (1.32%) and 0.20 mM sodium chloride (NaCl) (1.24%), demonstrating good selectivity.Moreover, the long-term stability of the PtNi 3 hydrogel for H 2 O 2 measurement was studied.The PtNi 3 hydrogel from one batch was used to measure 0.10 mM H 2 O 2 every 5 days.It can be seen from Fig. 3e that the catalytic activity was well maintained even after 60 days (RSD = 2.30%), indicating the outstanding longterm stability of the PtNi 3 hydrogel-based sensing platform.These sensing performances were better than those of most reported colorimetric H 2 O 2 sensors (see Table S2).Finally, we measured the released H 2 O 2 from the 3-[(3cholanidopropyl) dimethylammonio]-1-pro panesulfonate (CHAPS)-stimulated HeLa cells.From Fig. 3f, significant absorbance was observed after the addition of 0.50 μM CHAPS to the solution with HeLa cells (3.60 × 10 5 cells mL −1 ), while there was no change in absorbance in the same solution without HeLa cells.The H 2 O 2 concentration was calculated to be 2.08 μM, confirming the feasibility of developing an accurate and sensitive H 2 O 2 sensor.

Electrocatalytic activity and H 2 O 2 electrochemical sensing
We also investigated the electrochemical characteristics of the Pt-Ni hydrogels in catalyzing H 2 O 2 .Figure 4 depicts the cyclic voltammograms (CVs) of the pure Pt, PtNi, PtNi 3 , PtNi 5 and pure Ni hydrogel-modified electrodes in 0.   4f).This phenomenon can also be ascribed to the promotion of catalysis by the synergistic effect between Pt and Ni in the alloyed nanowires as well as the higher affinity provided by moderate Ni(OH) 2 nanosheets 40,41 , which has been discussed before.Thus, the PtNi 3 hydrogel was chosen for the subsequent experiments.Then, the analytical performance of the PtNi 3 hydrogelbased electrode for the electrochemical detection of H 2 O 2 was evaluated via the amperometric i-t curves (i-t curves).Due to the wide catalytic potential window of H 2 O 2 for the PtNi 3 hydrogel, −0.30V was selected as the optimized bias voltage due to the highest current response (see Fig. 5a) and the ability to avoid coexisting interferences.The results shown in Fig. 5b revealed that the current response increased rapidly with the concentration of H 2 O 2 .A high sensitivity of 172.82 μA mM −1 cm −2 was estimated within a wide linearity range of 0.50 µM-5.0 mM with an LOD of 0.15 µM (S/N = 3) according to the calibration curve given in Fig. 5c.Moreover, excellent selectivity was observed, as shown in Fig. 5d, as the common interfering substances (0.20 mM UA, 0.20 mM AA, 0.20 mM Glu, 0.20 mM KCl, 0.20 mM NaCl) had no significant current response with respect to 0.10 mM H 2 O 2 .Furthermore, we investigated the influence of pH on the electrocatalytic activity of the PtNi 3 hydrogel.The results shown in Fig. S9 indicated that the electrocatalytic performance of the hydrogel was also pH dependent.The catalyst performed well in the weakly acidic environment but poorly in the strongly acidic or alkaline conditions.This is because strong acid will corrode the Ni(OH) 2 nanosheets, while alkaline conditions will cause the decomposition of H 2 O 2 .Additionally, the longterm stability of the PtNi 3 hydrogel in catalyzing H 2 O 2 was also explored, and the result is given in Fig. 5e.The PtNi 3 hydrogel displayed consistent current responses in the 60day measurement (RSD = 3.48%), demonstrating its longterm stability.This outstanding stability could be explained by the following two reasons: (1) the PtNi 3 hydrogel applied as the sensing material avoided the problem of activity attenuation of the natural enzymes, thus enhancing the chemical stability; (2) the physical stability of the hydrogel was guaranteed by the dual structure of the interfused nanowires and nanosheets, further promoting the longterm stability.
The excellent sensing performance endows the PtNi 3 hydrogel with great application potential in complex biological systems.Therefore, we finally measured the H 2 O 2 secreted from HeLa cells stimulated by CHAPS.In the presence of HeLa cells (3.60 × 10 5 cells mL −1 ), an obvious current response was observed as soon as CHAPS (0.50 μM) was added (red line in Fig. 5f).The concentration of H 2 O 2 was calculated to be 1.84 μM according to the calibration curve in Fig. 5c.In contrast, there was no obvious current change under the same experimental conditions in the absence of HeLa cells (black line in Fig. 5f), confirming that the measured H 2 O 2 was generated from the cells.These results verified the further applicability of the PtNi 3 hydrogel in sensing living cells.

Portable visual and electrochemical H 2 O 2 sensors
Developing miniaturized, portable and cost-saving H 2 O 2 sensors will fulfill personalized health care needs, which has important practical significance and broad future application prospects in the future 42 .To take advantage of the colorimetric and electrochemical H 2 O 2 sensing strategies based on the excellent peroxide-like and electrocatalytic activities of the PtNi 3 dual hydrogel, we used it to construct dual portable visual and electrochemical sensors for H 2 O 2 .For the visual H 2 O 2 sensor, we first produced colorimetric test paper by dropping PtNi 3 hydrogel and TMB on filter paper (Fig. 6a, b).After integration with an M5stack development board and a color sensor, a portable visual H 2 O 2 platform was successfully constructed.When the sample containing H 2 O 2 was dropped onto the test paper, a color change was observed within 3 min.Then, this color signal was simultaneously captured and converted into R/G/B data by a color sensor.The microprocessor in an M5stack board can transform these data into the H 2 O 2 concentration and directly display the information on a screen.The calibration curve between the H 2 O 2 concentrations and the values of (B/(R + G + B)) at 25 °C was obtained as described in Table S3 and is shown in Fig. 6c, revealing excellent linearity in the concentration range of 0.50 µM-1.0 mM.It should be noted that the ambient temperature and humidity had little influence on the performance of the test paper (Figs.S10 and S11) due to its rapid response.Moreover, we designed a portable electrochemical H 2 O 2 sensor with the aid of an in-house built signal processing circuit and an M5stack board, which were connected to the PtNi 3 hydrogel-modified SPE (Fig. 6d, e).The current output of the SPE sensing chip was processed by the circuit and transmitted to the M5stack board.Then, the measured H 2 O 2 concentrations were obtained according to the calibration curve (see Fig. 6f) (linearity range: 0.50 µM-2.0 mM) and synchronously displayed on the screen.With the advantages of high integration, simple operation, portable detection and low cost, the as-fabricated visual and electrochemical sensors show promise for convenient H 2 O 2 determination without professional instruments and operators.
Considering the importance of H 2 O 2 analysis in living cells, we used these two kinds of portable sensors to measure the H 2 O 2 content in CHAPS-stimulated HeLa cells.For the colorimetric sensor, the measured H 2 O 2 concentration was 1.97 µM (see Fig. 6g) within 3 min, which coincided well with of the result obtained using a UV-vis spectrophotometer (2.08 µM) shown in Fig. 3f.For the electrochemical sensor, the content of H 2 O 2 secreted from the HeLa cells was measured as 1.77 µM in 6 s, which was also in good agreement with the value determined by an electrochemical workstation (1.84 µM) (Fig. 6h, i).These results demonstrated the reliability of our H 2 O 2 sensors in practical application.Moreover, to further verify the accuracy of the prepared sensors, we conducted comparative experiments in 0.1 PBS (pH 5.0) under different concentrations of H 2 O 2 with the portable colorimetric sensor, the UV-vis spectrophotometer, the portable electrochemical sensor, and the electrochemical workstation.The results shown in Table S4

Synthesis of the Pt-Ni hydrogels
A simple coreduction method was employed to prepare the Pt-Ni hydrogels 43 .Specifically, 46.80 μL of H 2 PtCl 6 solution (205.0 mM) and 2.88 mL of NiCl 2 solution (10.0 mM) were mixed with 39.50 mL of water and stirred for 10 min.Then, 0.050 mmol of NaBH 4 was added under vigorous stirring, and the color of the solution turned immediately from light yellow to dark brown.After 2 min of continuous stirring followed by standing for 24 h in a dark room, a PtNi 3 hydrogel with a Pt:Ni molar ratio of 1:3 was obtained.The prepared hydrogel was washed by removing and replacing the supernatant with deionized water 7 times.Finally, the concentration of the PtNi 3 hydrogel was adjusted to 0.050 μg (PtNi3) mL −1 and 1 mg (PtNi3) mL −1 for the subsequent colorimetric and electrochemical experiments.The PtNi and PtNi 5 hydrogels were synthesized by tuning the ratio of H 2 PtCl 6 to NiCl 2 accordingly.The molar mass of the added NaBH 4 was also adjusted with the amounts of the precursors.Pure Pt and Ni hydrogels were obtained through the same route by using H 2 PtCl 6 or NiCl 2 as the only metal precursors.

Construction of the dual portable H 2 O 2 biosensors
The PtNi 3 -TMB solution was first prepared by mixing 10.0 μL of PtNi 3 hydrogel (0.050 μg mL −1 ) and 10.0 μL of TMB (50.0 mM) into 1.0 mL of PBS (0.10 M, pH 5.0).Colorimetric test paper was fabricated by dropping 20.0 μL of PtNi 3 -TMB solution onto a round filter paper (Φ = 5.0 mm).Then, the test paper was fixed on a color sensor (30.0 mm◊23.0mm◊8.0 mm), which was integrated with a TCS3472 chipset.The color sensor was operated by shining an LED through the test paper, and the reflected light was absorbed by a 3◊4 array of photodiodes (3 had red filters, 3 had green filters, 3 had blue filters and 3 had no filter).Then, the generated photocurrent was converted into an RGB-related digital signal by the analog-to-digital conversion module and transmitted to the M5stack development board (53.0 mm◊53.0mm◊17.0mm), which is a portable and highly integrated platform including an ESP32 system as well as a built-in battery module.
For the portable electrochemical H 2 O 2 sensor, 3.0 μL of 3 hydrogel (1.0 mg mL −1 ) was modified on the SPE, in which a working electrode, a counter electrode and a reference electrode were integrated on a polyethylene glycol terephthalate (PET) film (30.0 mm◊9.0 mm◊0.20 mm) via the screen printing method.With the help of an in-house built signal processing circuit (30.0 mm◊25.0mm◊1.50 mm) as well as the M5stack board, this portable sensor can be used to detect the current responses toward H 2 O 2 from the PtNi 3 hydrogel-modified SPE (illustrated in Scheme 1).Then, the amplified current responses were transformed to digital signals and transmitted to the M5stack development board.
In the experiments on measuring H 2 O 2 released by living cells, 0.50 μM CHAPS was added to a solution with HeLa cells (3.60 × 10 5 cells mL −1 ).Then, H 2 O 2 was released from the stimulated HeLa cells and measured by the as-fabricated H 2 O 2 sensors.

Apparatus and measurement
SEM images were obtained from FEISEM (NANO-SEM450, USA) upon an accelerating voltage operating at 15.0 kV.TEM was carried out on an FEI Talos S-FEG (Thermo Scientific, USA).XRD was conducted on a CRD-7000 (Shimadzu, Japan), and XPS was performed on a Model K-Alpha (Thermo Fisher Scientific Company, USA).The UV-vis spectra data were recorded on a U-3900H spectrophotometer (Hitachi, Japan).Inductively coupled plasma-optical emission spectrometry (ICP-OES) was carried out on an ICAP7600DUO optical emission spectrometer (Spectro, Germany).
All electrochemical characterizations (i.e., CVs and i-t curves) were performed on a 660E electrochemical workstation (Shanghai Chenhua Instrument Corporation, China) with a single0compartment, three-electrode cell at room temperature.A Pt-Ni hydrogel-modified glassy carbon electrode (PtNi x /GCE) was used as the working electrode.A Ag/AgCl electrode and platinum wire were used as the reference electrode and the counter electrode, respectively.

Fig. 1
Fig. 1 Characterization of the PtNi3 hydrogel.a SEM, b TEM, c HRTEM, d elemental mapping images, e XRD pattern, f XPS wide scan, and highresolution (g) Pt 4 f and (h) Ni 2p spectra of the PtNi 3 hydrogel

Fig. 2
Fig. 2 Investigation of the peroxidase-like characteristics of the PtNi x hydrogels.a Typical absorption spectra of the PtNi 3 hydrogel in 0.50 mM TMB solution; b the measurement of •OH by the fluorescence absorption spectra in 0.50 mM TA solution.The concentrations of H 2 O 2 and the PtNi 3 hydrogel were 0.50 mM and 0.050 μg mL −1 , respectively; c typical absorption spectra of the Pt, Ni, and PtNi x hydrogels in 0.10 M phosphate-buffered saline (PBS) (pH 5.0) with 0.50 mM TMB and 0.20 mM H 2 O 2 ; d the absorbance peaks of the Pt, Ni, and PtNi x hydrogels at 652 nm; (e, f) comparisons of the K m and K cat of the PtNi x , pure Pt, Ni hydrogels and other artificial/natural enzymes

Fig. 5
Fig. 5 Analytical performance of the PtNi 3 hydrogel in H 2 O 2 electrochemical sensing.a The i-t curves of PtNi 3 /GCE at bias voltages of −0.10 V, −0.20 V and −0.30V, respectively; b the i-t curve recorded at PtNi 3 /GCE with the successive addition of H 2 O 2 under a bias voltage of −0.30V and (c) the corresponding calibration curve; (d) the anti-interference experiment with 0.10 mM H 2 O 2 and 0.20 mM interferences; (e) the current response of 0.10 mM H 2 O 2 at PtNi 3 /GCE for 60 days; (f) the i-t curves of PtNi 3 /GCE responding to the H 2 O 2 released from HeLa cells upon CHAPS stimulation

Fig. 6
Fig. 6 Diagram and application of the portable visual and electrochemical H 2 O 2 sensors.a-c Schematic illustration and corresponding calibration curve of portable visual H 2 O 2 sensing based on testing paper.d-f Schematic illustration and corresponding calibration curve of portable electrochemical H 2 O 2 sensing.g, h Measurement of H 2 O 2 released from HeLa cells with portable visual and electrochemical sensors.Scale bar = 1.0 cm.i Comparison of H 2 O 2 concentrations measured with the portable colorimetric sensor, the UV-vis spectrophotometer, the portable electrochemical sensor, and the electrochemical workstation, respectively The optimized PtNi 3 hydrogel-based H 2 O 2 sensing platforms displayed remarkable performance in both colorimetric and electrochemical methods with wide linearity ranges, low limits of detection (LODs), robust long-term stability and good selectivity against common interferences.Together with an M5stack development board, a portable visual H 2 O 2 sensor with an integrated PtNi 3 hydrogel-based colorimetric test paper and a portable electrochemical H 2 O 2 sensor with an integrated PtNi 3 -modified screen-printing electrode (SPE) were successfully constructed.Finally, accurate quantitative analysis of H 2 O 2 released from HeLa cells was achieved, revealing the practicability of the proposed biosensors.This work not only demonstrates the versatility of metal hydrogels but also provides a new strategy for highperformance and low-cost sensor design.
. Herein, we developed portable H 2 O 2 sensors based on Pt-Ni hydrogels composed of alloyed nanowires and Ni(OH) 2 nanosheets, which have demonstrated unexpected peroxidase-like and electrocatalytic properties toward H 2 O 2 .
10 M PBS (pH 7.40) containing 0.20 mM H 2 O 2 .The reduction currents of the Pt, PtNi, PtNi 3 , and PtNi 5 hydrogels all clearly increased with the addition of H 2 O 2 , while the pure Ni hydrogel showed almost no response, demonstrating the high activities of the Ptbased hydrogels in the electrocatalytic reaction of H 2 O 2 .
indicated that the developed H 2 O 2 sensors maintained high accuracy at different sample concentrations.In addition to the advantages of high integration and portable detection, electrochemical sensors can enable faster and more accurate H 2 O 2 determination.In summary, this work demonstrated portable H 2 O 2 sensors based on a multifunctional PtNi 3 hydrogel, which exhibited superior peroxidase-like and electrocatalytic activities toward H 2 O 2 .The favorable catalytic properties of the 3D porous PtNi 3 hydrogel benefited from the unique dual structures of alloyed Pt-Ni nanowires and moderate Ni(OH) 2 nanosheets, which significantly enhanced the catalytic efficiency and the reaction affinity, respectively.Based on this, the colorimetric and electrochemical H 2 O 2 sensing platforms displayed excellent analytic performances, such as wide linearity ranges (0.10 μM-10.0mM & 0.50 μM-5.0 mM), low LODs (0.030 μM & 0.15 μM), and outstanding long-term stability for as long as 60 days.Ultimately, portable visual and electrochemical H 2 O 2 sensors were successfully constructed and applied to the determination of H 2 O 2 released from HeLa cells with the aid of sensing unit (test-paper and SPE) signal transformation (color sensor and signal processing circuit) and an M5stack development board.This work not only proves the great potential of versatile metallic hydrogels in H 2 O 2 sensing but also introduces a new approach for the development of portable sensing devices in practical applications.