Introduction

Dopamine (DA) acts as a neurotransmitter in the central nervous system, impacting physiological and neurological activities [1]. The abnormal levels of DA are able to trigger various neurological diseases, such as schizophrenia [2], cardiovascular disease [3], and Alzheimer’s disease [4]. Fortunately, the relative pathological function can release a certain concentration of small biological molecules including H2O2, glucose, and DA. Thus, we can detect these small biological molecules as metabolic parameters for promising disease diagnosis. Accordingly, accurate quantitative determination of DA is vitally essential for clinical analysis and early diagnosis of relative diseases.

Nevertheless, the lack of electrical activity, chromophores, and fluorophores makes the detection of DA become particularly difficult. In order to efficiently detect DA, various biosensors such as electrochemistry [5], chemiluminescence, colorimetry [6], and fluorescence [7] have been developed. Among these methods, the colorimetric method has been focused on due to the unique advantages of sensitive reaction, simple preparation, intuitive, good selectivity, and low requirements on the instrument [8]. Accordingly, it is necessary to establish a novel material for DA and apply it conveniently in practice.

Nanozymes, a class of simulated enzymes with unique properties of nanomaterials, have attracted more and more attention due to their advantages in economy, stability, and catalytic activity over natural enzymes. Recently, various nanozymes including metal oxides [9], metal sulfides [10], and precious metal nanoparticles [11] have been verified to possess good peroxidase activity. Compared with single-metal nanomaterials, bimetallic nanomaterials show excellent catalytic performance. Chen et al. developed CeCoO3 bimetallic nanomaterials and successfully applied them to sensitive detection of glutathione. Although many nanozymes have been explored and developed, their catalytic mechanism, sensitivity, and selectivity are still important challenges in research. Among strategies of developing the catalytic activity of nanoenzymes, organic molecules with large conjugate macrocycles have been considered attractive ones to modify some semiconductor nanomaterials [12]. Porphyrins (Por) are such ones. Porphyrins are usually used as photosensitizers to enhance the performance of some inorganic nanomaterials applied in the solar cells [13], photodynamic therapy [14], and catalysis [15], etc. Moreover, some merits including good biocompatibility and strong coordination with metals make porphyrins become ideal candidates to functionalize inorganic nanoenzymes and develop catalytic activity, accordingly [16]. Therefore, considering an interesting spinel cobaltite Por-CuCo2O4 as an excellent peroxidase, a fast colorimetric sensing platform for DA can come true.

Herein, we prepared 5,10,15,20-tetrakis (4-carboxyl phenyl) porphyrin (Fig. S1)–functionalized urchin-like CuCo2O4 nanocomposites (Scheme 1), which were verified to possess an excellent peroxidase-like activity with the aid of TMB accompanied by a color change in the process of the catalytic reaction. The catalytic performance of Por-CuCo2O4 peroxidases conforms to the Michaelis-Menten equation. Notably, the response time of blue color (oxTMB) is 10 s, a very short time, which is very important to construct a fast colorimetric sensor to realize real-time detection. Electrochemistry, radical scavenger, and fluorescence probe experiments indicate that electron transfer, •O2− radicals, and holes (h+) are the important factors during the catalytic oxidation of TMB. In addition, based on Por-CuCo2O4 peroxidases, a fast colorimetric sensing platform for H2O2 and DA was developed with high sensitivity and good selectivity.

Scheme 1
scheme 1

Schematic illustration for the formation process of urchin-like Por-CuCo2O4 nanospheres

Full size image

Experiment and methods

Reagents

Copper chloride dihydrate (CuCl2·2H2O), hydrogen peroxide (H2O2, 30 wt.%), cobalt chloride hexahydrate (CoCl2·6H2O), and dopamine hydrochloride were commercially provided by Aladdin Biochemical Technology Co., Ltd. 3,3′,5,5′-tetramethylbenzidine dihydrochloride (TMB•2HCl) was procured from Macklin (Shanghai, China). Ethylene diamine tetraacetic acid disodium salt (EDTA), urea, Na+ (NaCl), K+ (KCl), Mg2+ (MgSO4), p-benzoquinone (PBQ), sucrose, lactose, fructose, d-histidine, l-arginine, d-serine, and isopropyl alcohol (IPA) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). The synthesis of 5,10,15,20-tetrakis (4-carboxyl phenyl) porphyrin (H2TCPP) was referred to by an earlier report [17].

Characterization

The products were analyzed by a powder X-ray diffractometry (XRD) instrument (Cu-Kα, λ = 1.54178 Å) and a scanning electron microscopy instrument (APREO, American) operated at an accelerating voltage of 2 kV and is equipped with an energy-dispersive X-ray spectroscopy (EDX) instrument. The valence state of the elements in Por-CuCo2O4 was analyzed by an X-ray photoelectron spectroscopy (XPS) instrument (Thermo ESCALAB 250 Xi). The specific surface areas and pore size distribution of nanoparticles were analyzed by the Brunauer-Emmett-Teller surface areas (BET) and the Barrett-Joiner-Halenda (BJH) process with a Micromeritics ASAP 2460 analyzer. Fluorometric data (FL) and the UV-vis absorption spectrum data were obtained on a Hitachi F-4600 spectrofluorophotometer (Japan) and the TU 1810 spectrophotometer (Puxi, China), respectively. The electrochemical performance is characterized by CHI 760E Chen-Hua electrochemical workstation.

Synthesis of urchin-like Por-CuCo2O4

Firstly, urchin-like CuCo2O4 microspheres were prepared by the hydrothermal method [18]. After that, the urchin-like Por-CuCo2O4 microspheres were prepared by a two-step method. The formation process of urchin-like Por-CuCo2O4 nanospheres is shown in Scheme 1. The detailed preparation was presented in supporting information.

After calcination, 3 mg H2TCPP was dissolved in NaOH alkaline water (pH = 9). After that, 60 mg of CuCo2O4 sample was added to the above solution, which was ultrasonically dissolved and transferred to a Teflon-lined autoclave with 110 °C hydrothermal for 60 min. After the sample was collected by centrifugation and further purified, Por-CuCo2O4 composite has been successfully synthesized.

Peroxidase-like activity of Por-CuCo2O4 and the DA detection

The experiment was conducted in acetic acid buffer containing Por-CuCo2O4 (0.015 mg mL−1), H2O2 (25 mM), and TMB (0.1 mM). The visible absorption spectra were collected after the system reacted for 90 s. Kinetic experiments were carried out by measuring the absorbance at 652 nm in acetate buffer containing Por-CuCo2O4, H2O2, and various concentrations of TMB at room temperature. Similarly, as a control experiment, other conditions remain unchanged as described above, by changing the H2O2 concentration and fixing the TMB concentration. The apparent kinetic parameters were calculated by the following equation: 1 ∕ v = (Km ∕ Vmax) × (1 ∕ [S]) + 1 ∕ Vmax, where v, [S], Vmax, and Km stand for initial velocity, the substrate concentration, the maximal velocity, and the Michaelis constant, respectively.

The DA detection is described in detail as follows: freshly prepared various concentrations of DA (10–700 μM) were added into the reaction mixture containing the buffer solution acetate (pH 4.0), Por-CuCo2O4 (0.015 mg mL−1), H2O2 (25 mM), and TMB (0.1 mM). The entire reaction mixture was incubated for 3 min, and the absorbance of various DA concentrations was recorded.

Test of active species

A standard three-electrode system (Por-CuCo2O4-modified glassy carbon electrode, the platinum wire electrode, and the saturated calomel electrode) was used in the electrochemical experiment. In the cyclic voltammetry experiment, the current and potential responses were recorded before and after the addition of H2O2 to form a control group. In the amperometric testing experiment, the electrodes of uncoated and coated materials were subjected to a control test, and the electrochemical response was collected after adding the H2O2 (1 M) to react for 60 s. H2O2 (1 M) was added to PBS every 50 s, and the electrochemical response of the solution was recorded.

The capture experiments were conducted by selecting EDTA, IPA, and PBQ to capture holes (h+), hydroxyl radicals (•OH), and superoxide radicals (•O2−), respectively. Specifically, different scavengers (200 μL) and Por-CuCo2O4 (100 μL) were injected into 2 mL of H2O2-TMB system to form a mixture. The absorbance at 652 nm was recorded after the above mixture reacted for 90 s at ambient temperature.

Fluorescent experiments were implemented by selecting terephthalic acid (TA) to capture •OH to form dihydroxyterephthalic acid (HOTA) with strong fluorescence. Specifically, freshly prepared various samples contain 5 mM TA, 25 mM H2O2, and Por-CuCo2O4 at a concentration of 0.1–0.8 mg mL−1 and acetate buffer at pH 4. Fluorescence spectra were recorded after the above mixtures were reacted for 30 min at the optimal temperature.

Detection of DA

The human serum samples of two volunteers were provided by the Affiliated Hospital of Qingdao University, China. To avoid the interference of coexisting substances, the resulting serum sample was diluted 80 times with PBS, which diluted DA to a certain concentration, and then added into the system containing H2O2-TMB. After 3 min of reaction, the absorbance of the reaction system at 652 nm was recorded.

Results and discussion

Characterization of Por-CuCo2O4 spinel microspheres

The phase of such interesting hierarchical microspheres was studied by XRD (Fig. 1a). The diffraction peaks at 19.07°, 31.36°, 36.96°, 38.96°, 45.07°, 56.03°, 59.60°, 65.70°, and 77.55° are indexed to the crystal plane of (111), (220), (311), (222), (400), (422), (511), (440), and (533) of cubic CuCo2O4 phase (JCPDS card no. 01-1155) [19]. Carefully, one diffraction peak at 35.55° is attributed to monoclinic CuO (JPCDS card no. 05-0661) [20]. Therefore, the main composition of nanocomposites is CuCo2O4, accompanied by trace amounts of CuO [21], because CuCo2O4 has poor thermal stability and is easy to produce CuO [22]. Notably, the XRD peaks of Por-CuCo2O4 are almost consistent with that of CuCo2O4, suggesting that the introduction of porphyrin has no effect on the lattice structure of the composites, due to a small quantity of porphyrin in the composites.

Fig. 1
figure 1

XRD (a) and XPS spectra data of the as-prepared CuCo2O4 and Por-CuCo2O4 nanocomposites: Survey scan spectra (b), Cu 2p (c), Co 2p (d), O 1s (e), and N 1s (f), respectively

Full size image

XPS is used to further study the composition and oxidation states of Por-CuCo2O4, shown in Fig. 1b–f. The survey spectra in Fig. 1b display the composition of Cu, Co, O, and N elements in CuCo2O4 and Por-CuCo2O4, respectively. In the Cu 2p spectra (Fig. 1c), two major spin-orbit doublets with binding energy at approximately 935.1 and 933.7 eV are ascribed to Cu2+ and Cu+ [23]. The Gaussian fitting curves show that the Co 2p spectra (Fig. 1d) are composed of Co 2p3/2 and Co 2p1/2 peaks at 935.1 and 933.7 eV with two satellite peaks located at approximately 941.2 and 943.8 eV [24]. As seen from Fig. 1e, the O1s spectrum can be fitted to three Gauss peaks at 532.7 eV, 531.3 eV, and 529.8 eV, which are attributed to chemisorbed oxygen, the oxygen vacancy, and the lattice oxygen [25], respectively. The existence of oxygen vacancy is in favor of the improvement of the catalytic activity of Por-CuCo2O4. Figure 1f shows the comparison spectra of N 1s in CuCo2O4 and Por-CuCo2O4. Compared with that of CuCo2O4, N 1s in Por-CuCo2O4 has peaks at bond energy of 398.5 eV and 400.4 eV, corresponding to the -N- bond and the -N = bond of the porphyrin ring [17], indicating that the porphyrin was successfully introduced.

Morphologies of CuCo2O4 and Por-CuCo2O4 were analyzed by SEM and TEM, respectively. As displayed in Fig. 2a and b, CuCo2O4 and Por-CuCo2O4 all exhibit urchin-like microsphere structure with 5–10 μm in size, which are composed of needle-like nanorods. Carefully, TEM images (Fig. 2c and d) display that needle-like nanorods are further composed of a lot of nanoparticles. From the HRTEM image of Por-CuCo2O4 (Fig. 2e), the lattice fringes of 0.456 nm, 0.284 nm, and 0.243 nm are in accordance with the crystalline planes of (111), (220), and (311) of CuCo2O4 in XRD data, respectively. Subsequently, the EDX element mapping (Fig. 2f) shows that Cu, Co, O, and N elements uniformly distribute Por-CuCo2O4 composite, indicating the successful preparation of Por-CuCo2O4. Additionally, the calculated BET specific surface area (Fig. S2a) of Por-CuCo2O4 composite material is 45.57 m2 g−1, which is much larger than pure CuCo2O4 (36.56 m2 g−1). In addition, the Barrett-Joiner-Halenda (BJH) model calculates that the Por-CuCo2O4 pore size distribution is about 2 nm and 6.9 nm in diameter (Fig. S2b). It can be seen that the introduction of porphyrin further increases the specific surface area of Por-CuCo2O4 and provides more active sites.

Fig. 2
figure 2

SEM images of CuCo2O4 (a) and Por-CuCo2O4 (b), respectively. TEM images of CuCo2O4 (c) and Por-CuCo2O4 (d, e). EDX mapping images (f) of elements of Por-CuCo2O4: Cu, Co, O, and N

Full size image

Peroxidase-like activity

In order to validate the peroxidase-like activity of Por-CuCo2O4, six reaction systems were devised as control experiments using TMB as the chromogenic substrate, which can be oxidized to generate blue oxTMB with a distinct absorption at 652 nm. As seen from curves 3c–3f in Fig. 3, four systems hardly have absorption in the absence of H2O2 or composites (CuCo2O4, Por-CuCo2O4), suggesting that either individual H2O2 or nanocomposite cannot trigger the oxidation reaction of TMB. Interestingly, systems a and b (curves 4a and 4b) have stronger absorption intensity accompanied by a marked color change (illustration), indicating that both CuCo2O4 and Por-CuCo2O4 possess the peroxidase-like activity. Moreover, the absorption intensity of Por-CuCo2O4 (system a) is more than twice that of CuCo2O4 (system b), which indicates that Por-CuCo2O4 has a stronger peroxidase-like activity. Therefore, the simultaneous existence of Por-CuCo2O4 peroxidases and H2O2 is the basic condition for the rapid oxidation of TMB. The reason why Por-CuCo2O4 has more excellent peroxidase-like activity is ascribed to a lot of oxygen vacancies verified by XPS as well as the synergetic effect between CuCo2O4 and H2TCPP molecules, which is further explained in the catalytic section, subsequently. The according color of different reaction systems can be quickly distinguished in 90 s, shown in the inset of Fig. 3. As we know, the catalytic activity of artificial peroxidase and natural peroxidase is influenced by pH and temperature. Figure S3 shows that pH = 4 and 45 °C are the best conditions, which will be applied to subsequent experiments.

Fig. 3
figure 3

UV-visible absorption spectra and corresponding photograph of color change of different reaction systems in 90 s

Full size image

The steady-state kinetics of Por-CuCo2O4 was studied by keeping the TMB concentration constant while changing the H2O2 concentration (and vice versa). Figure 4a and c respectively show the relationship between the concentration of H2O2 and TMB as substrates and the initial reaction rate. The values of Km and Vmax were calculated according to the Michaelis-Menten equation (Fig. 4b and d). In comparison with the affinity of HRP together with other artificial peroxidase, some related parameters are shown in Table S1. Compared with that of HRP and other artificial peroxidase (Cu(OH)2, Co3O4@CeO2, PtCNPs, [Cu(PDA)(DMF)]), our obtained Por-CuCo2O4 has the lower Km value, indicating that Por-CuCo2O4 has a stronger affinity towards TMB, which is a favorable factor for catalytic enhancement of Por-CuCo2O4.

Fig. 4
figure 4

Steady-state kinetic tests of Por-CuCo2O4 using Michaelis-Menten model. a The concentration of TMB was 100 μM and varying the H2O2 concentration from 1 to 10 mM. b The corresponding double reciprocal curves for H2O2. c The concentration of H2O2 was 35 mM and varying the TMB concentration from 10 to 100 μM. d The corresponding double reciprocal curves for TMB. Error bars represent the standard deviation for three measurements. The data was collected at 652 nm

Full size image

Colorimetric detection of small biological molecules and selective exploration

As an oxidant, H2O2 is not only used in industrial production but also plays a key role in biological processes. However, long-term excessive use of H2O2 can cause diseases such as low immunity, genetic mutations, arteriosclerosis, and diabetes [26]. Therefore, designing a fast colorimetric sensing platform for H2O2 is particularly important. Herein, based on Por-CuCo2O4 with higher peroxidase-like activity, a series of experiments (Fig. 5a) were designed and implemented by changing the concentration of H2O2 (0.1–10 mM). From the data, a very good linear relationship (inset) of absorbance and H2O2 concentration in the concentration range of 0.1–1.0 mM (R2 = 0.998) was obtained. The detection limit (LOD) is calculated to be 90.26 μM (LOD = 3 s/k), suggesting a good sensitivity of the constructed colorimetric sensor based on Por-CuCo2O4 to the TMB-H2O2 system.

Fig. 5
figure 5

Dose-response curve for H2O2 (a) and or DA (b) determination at 652 nm. b ΔA response of H2O2-TMB system towards different DA concentrations from 10 to 700 μM; the inset is the linear fitting curve from 10 to 100 μM. The variance of net absorbance (ΔA) in the presence of various substances. ΔA = A0 − A, where A0 and A are the absorbance of the Por-CuCo2O4 system at 652 nm in the presence and absence of DA. c The selectivity of the Por-CuCo2O4 towards H2O2 (0.1 mM) and interferents (1.0 mM). d The selectivity of the Por-CuCo2O4 towards DA (1.0 mM) and interferents (10 mM). Error bars denote standard deviations based on three measurements

Full size image

As a neurotransmitter, DA as a necessary substance for our body can regulate various physiological functions of the central nervous system. Therefore, a simple method for rapid determination of DA is meaningful. Therefore, a series of experiments were carried out by tuning DA concentration (10–700 μM) under the optimal conditions (pH = 4 and 45 °C). Because phenol hydroxyl and amino groups are attached in the molecule structure of DA, DA molecules exhibit reducibility. Thus, in our designed sensing system (DA + Por-CuCo2O4 + H2O2 + TMB), DA molecules can prevent TMB from being oxidized by H2O2, resulting in a reduction of the oxTMB absorbance together with fading of blue color visually [27]. The response curve of DA concentration and absorbance difference value ΔA (ΔA = Ablank − ADA) has a good linear relationship at 10–100 μM (inset of Fig. 5b). In addition, compared with that of DA measured by different methods (colorimetry, fluorimetry, and electrochemistry) in different catalyst systems (Co3O4@NiO, CeO2, Pt/CoSn(OH)6, ZnO@Cys NPs, Pdots@AMP-Cu, RGO-ZnO, CRGO-Au NCs, Ag/rGO) listed in Table S2, a lower LOD (0.94 μM, S/N = 3) of the Por-CuCo2O4-based colorimetric sensor was found. As we know, electrochemistry method is known as high sensitivity. Expectably, our designed colorimetric sensing platform for DA was on a par with the electrochemistry method.

Other than high sensitivity, good selectivity is also necessary for colorimetric sensors. From Fig. 5c and d, absorbance difference value ΔA of two systems is the highest and hardly influenced in the presence of common interfering molecules (Suc, Fru, Lac Arg, Ser, His, Na+, K+, Mg2+, UA, ISO, Leu), even though their concentration is 10 times than that of H2O2 and DA, respectively. Nevertheless, this method is still a big challenge for distinguishing DA and materials with reducing substances. It is suggested that the colorimetric sensing platform has good selectivity for the determination of H2O2 and DA in the absence of reducing substances.

Proposed mechanism

According to publications, the catalytic mechanism of artificial peroxidases is ascribed to two kinds as follows: one is from direct electron transfer between the substrates and reactants. The other is due to some active species during the catalytic reaction. Therefore, the possible catalytic mechanisms need to be verified one by one. Firstly, an electron transfer process was studied by electrochemical methods (cyclic voltammetry and amperometric methods). In the cyclic voltammetry experiment (Fig. 6a), the Por-CuCo2O4 modified electrode (control group) has no obvious current in the absence of H2O2. Nevertheless, after adding 1 M H2O2, a clear current response with an obvious redox peak at −0.45 eV was obtained, indicating that Por-CuCo2O4 has the ability to transfer electrons between the electrode surface (electron donor) and H2O2 (electron acceptor) [28]. At −0.45 eV, the ampere experiment of Por-CuCo2O4 catalyzed bare GCE and Por-CuCo2O4 modified GCE (Por-CuCo2O4/GCE) is shown in Fig. 6b. Adding H2O2 every 50 s, the reduction current in the Por-CuCo2O4/GCE electrode increases steadily with a high sensitivity. The data suggest that Por-CuCo2O4 peroxidases can speed up electron transfer.

Fig. 6
figure 6

a Cyclic voltammetry of Por-CuCo2O4/GCE in 40 mL PBS in the absence (blank) and in the presence (red) of 1 M H2O2 (scan rate: 0.05 V/s). b The typical steady-state current response of the Por-CuCo2O4/GCE to the successive addition of H2O2 (1 M) every 50 s into the PBS under stirring at the applied potential of −0.45 V. c Effects of various active scavengers during the catalysis of TMB with the aid of Por-CuCo2O4. d Fluorescence intensity varies with the concentration of Por-CuCo2O4 at different wavelengths. e The UV-vis DRS spectra and f Tauc plots of CuCo2O4

Full size image

Secondly, to study the possible active species produced in the process of Por-CuCo2O4 catalytic reaction, various capture experiments were implemented. Based on the previous results and our constructed colorimetric sensing system, three active species, namely, holes (h+), superoxide radicals (•O2−), and hydroxyl radicals (•OH), are produced, and the catalytic system may exist during the catalytic reaction. The active species including h+, •O2−, and •OH can be captured by trapping agents PBQ, EDTA, and IPA, respectively. If three active species are produced during the catalytic reaction, the absorbance of the catalytic system will be decreased by addition of the trapping agents. Obviously, as seen from Fig. 6c, •O2− and h+ play an important role while •OH is not detected by the trapping agent method. In order to further determine whether the catalytic oxidation of TMB by Por-CuCo2O4 nanospheres is based on the hydroxyl (•OH) mechanism, terephthalic acid (TA) was selected as the trapping agent. It is known that the resulting •OH reacts with TA, a non-fluorescent molecule, to form highly fluorescent hydroxy terephthalate acid (HTA), which shows an emission maximum at 432 nm when excited at 315 nm [29]. Figure 6d shows the relationship of the fluorescence intensity and Por-CuCo2O4 concentration. As seen from the figure, the fluorescence intensity is decreased with increasing of Por-CuCo2O4 concentration (10–80 μg/mL), further suggesting that •OH does not exist during the catalytic reaction. The fluorescence result is consistent with the capture experiment. Therefore, based on the results, the catalytic mechanism of Por-CuCo2O4 is not only from the direct electron transfer between reactants but also from two active species (•O2− and h+) produced in the course of the catalytic reaction.

According to the UV-vis diffuse reflectance spectra and Tauc plots (Fig. 6e and f), the forbidden bandwidth (Eg) of the CuCo2O4 is determined to be 1.56 eV [30]. Through the following Mulliken electronegativity theory formula (E0 = 4.5 eV, X is the geometric mean of the absolute electronegativity of the constituent atoms in CuCo2O4), the valence band position (VB) and conduction band (CB) position of CuCo2O4 are located at 2.24 and 0.68 eV, respectively [31]. The position of the conduction band valence band of H2TCPP refers to previous publications [32].

$$ {\displaystyle \begin{array}{l}{E}_{\mathrm{VB}}=X-{E}_0+0.5{E}_{\mathrm{g}}\\ {}{E}_{\mathrm{CB}}={E}_{\mathrm{VB}}-{E}_{\mathrm{g}}\end{array}} $$

Based on the mentioned information, a catalytic mechanism of Por-CuCo2O4 is proposed (Scheme 2). As we know, porphyrin molecules have absorption in the visible region and are usually considered photosensitizers. Thus, the electrons on HOMO can jump into LUMO of the porphyrin under visible light, resulting in the production of h+. At the same time, the unstable electrons on LUMO of porphyrin are further rapidly transferred to the CB of CuCo2O4. TMB adsorbed on the surface of Por-CuCo2O4 due to the accumulation effect provides the lone electron pair in the amino group to Por-CuCo2O4, increasing its electron density and mobility [33]. After that, the electron-rich Por-CuCo2O4 center can make electrons be transferred to H2O2, thereby reducing it to oxygen [34]. Furthermore, the transferred electrons reduced O2 into •O2− radicals, due to the a lot of oxygen vacancies that existed in Por-CuCo2O4. In addition, due to the quantum size effect, the large specific surface area, and small crystal size, Por-CuCo2O4 composed of a lot of smaller nanoparticles can provide much more catalytic active sites, thereby binding substrates with smaller steric hindrance and stronger redox capacity [33]. In addition, based on the reducing property of DA (which can fade blue oxTMB), sensitive and selective detection of DA can come true.

Scheme 2
scheme 2

Schematic illustration of Por-CuCo2O4 as a peroxidase mimic for H2O2 and dopamine detection

Full size image

Determination of DA in serum

Before determination, the DA is dissolved into the serum to make a standard solution and then diluted to different concentrations within the linear range. The accuracy of this method can be measured by the recovery and relative standard deviation (RSD) between the determined concentration and the standard concentration [35]. As displayed in Table 1, the recovery rate and RSD are in the range of 88.64–109.34% and 0.91–3.67%, respectively. Therefore, the recoveries are acceptable, and the Por-CuCo2O4-based colorimetric sensing platform can be used to determine DA in real samples, which has a good application prospect in biological monitoring.

Table 1 Results of dopamine detection in human serum samples
Full size table

Conclusions

A novel urchin-like Por-CuCo2O4 with enhanced peroxidase-like activity was successfully synthesized by a simple hydrothermal method. Due to the synergistic effect between H2TCPP molecules and CuCo2O4 under visible light, a large amount of active free radical species (•O2− and h+) was produced and improved to the catalytic activity of Por-CuCo2O4. Even in a wide temperature range (35–55 °C), the relative activity of Por-CuCo2O4 is still higher than 90%. The TMB + Por-CuCo2O4 colorimetric sensing platform has high selectivity and good sensitivity and can accurately determine H2O2 and DA, and its detection limits are 90.26 μM and 0.94 μM, respectively. In addition, the determination of DA in serum has achieved satisfactory results. We believe that this work will help the design of other artificial mimic with enzyme activity and be used for real-time monitoring of certain key biomolecules related to H2O2 in the fields of medicine and food.