Adaptable graphitic C6N6-based copper single-atom catalyst for intelligent biosensing

Self-adaptability is highly envisioned for artificial devices such as robots with chemical noses. For this goal, seeking catalysts with multiple and modulable reaction pathways is promising but generally hampered by inconsistent reaction conditions and negative internal interferences. Herein, we report an adaptable graphitic C6N6-based copper single-atom catalyst. It drives the basic oxidation of peroxidase substrates by a bound copper-oxo pathway, and undertakes a second gain reaction triggered by light via a free hydroxyl radical pathway. Such multiformity of reactive oxygen-related intermediates for the same oxidation reaction makes the reaction conditions capable to be the same. Moreover, the unique topological structure of CuSAC6N6 along with the specialized donor-π-acceptor linker promotes intramolecular charge separation and migration, thus inhibiting negative interferences of the above two reaction pathways. As a result, a sound basic activity and a superb gain of up to 3.6 times under household lights are observed, superior to that of the controls, including peroxidase-like catalysts, photocatalysts, or their mixtures. CuSAC6N6 is further applied to a glucose biosensor, which can intelligently switch sensitivity and linear detection range in vitro.

previously reported literatures. 3   When the light was on, the incubation was performed under a household white LED (=400-900 nm, 50 mW/cm 2 ).

Synchronous illumination X-ray photoelectron spectroscopy (SI-XPS). The SI-XPS
tests were performed on a XPS instrument (ESCALAB 250Xi) equipped with a 300 W Xe arc lamp as illumination source for providing the simulated solar-light with full spectrum. During the measurements process, the changes of XPS spectra were recorded by controlling light on or off at given time intervals. The samples were dispersed in water, deposited on substrates, and dried for 12 h. After that, the obtained samples were put in pretreatment chamber of XPS equipment for 12 h to remove the physically absorbed water molecules. Finally, the samples were transferred to the analysis chamber to perform the SI-XPS measurements.
Computational methods. Density functional theory calculations were performed using Gaussian 16 (revision C.02). 6 The ground-state geometries were optimized at a typical M06-2X/6-31G(d) level. All the optimized structures were real minima on the potential energy surface by means of frequency calculations. For the structure model anchored with/without a Cu atom, the charge of the system was set to +1/ 0 according to the experimental results, and the spin multiplicity was set to 1. Based on the optimized structures, electron excitations were calculated by means of the time-dependent density functional theory method. For this purpose, the first 50 excited states were calculated using M06-2X (54% Hartree-Fock function) 7 with def2-TZVP basis set in the gas phase. 8 To evaluate the reliability of the M06-2X hybrid functional in our system, the ground-state geometries (the 6-31G(d) basis set) and electron excitation (the def2-TZVP basis set) using PBE0 (25% Hartree-Fock function) 9, 10 and ωB97XD (22.2% Hartree-Fock function) 11,12 hybrid functionals were also calculated ( Supplementary Fig.   40), which gave the similar results.
The absorption spectra, the excited state of the electron-hole distribution 13      As shown in Supplementary Fig. 18, a much smaller impedance of CuSAC6N6 was As shown in Supplementary Fig. 19, the PL intensity of CuSAC6N6 was virtually invisible in comparison with Cu-N-C/CNmw and PCN, indicating an increased amount of non-radiative transition presumably by multiple pathways of charge transfer. As shown in Supplementary Fig. 23 and Supplementary Fig. 24, the temperature of reactors was tuned using a heater to the same value as that by light irradiation. The enhancement of peroxidase-like activity was negligible under the same increase of temperature, confirming the photothermal-induced gain effect here was marginal. NBT, a widely used probe for the detection of SOD-like activity, could be specifically reduced by O2 ·to produce a wide absorption spectrum from 450 nm to 700 nm (typically centered at 550 nm). 17 As shown in Supplementary Fig. 30, the absorbance of NBTre at 550 nm was negligible under the basic and gain reaction, confirming the generation of O2 ·in catalytic oxidation of TMB process here was marginal. Since coumarin has no absorption of the excitation light, this molecule was employed to generate the fluorescent product for detecting ·OH radicals in solution. 18 As shown in Supplementary Fig. 31, the fluorescence emission peak of umbelliferone at 447 nm was remarkable, confirming the existence of ·OH in catalytic oxidation of ABTS under the gain reaction. For a better scholarly presentation, the structure of CuSAC6N6 and the control C6N6 have been additionally calculated. As shown in Fig. 5b and Supplementary Fig. 37a, owing to the acceptor was changed from a =N-Cu-N= to two N atoms, the contribution of D-A in the CTS ( Fig. 5c and Supplementary Fig. 37b) decreased from 9.6% (CuSAC6N6) to 2.2% (C6N6). It was worth noting that the existence of Cu atom in CuSAC6N6 broke the symmetry, demonstrating that CuSAC6N6 accepted electrons more readily than C6N6 that were excited from three triazine rings (donor). To comprehensive analysis of the computational description for delocalization, the CTS of CuSAC6N6 with four possible metal-anchoring positions were calculated. In these structures, Cu was anchored to two N atoms at the central/edge triazine ring. As shown in Fig. 5b, Supplementary Fig. 38b, 38d, and 38f, the contribution of D-A transfer transition in the CTS were 9.6%, 3.8%, 6.3%, and 7.6%, respectively, much higher than that without Cu anchored (Supplementary Fig. 37b To comprehensive analysis of the computational description for conformational flexibility, five typical structures were randomly extracted from the optimization process (Supplementary Fig. 39a). The energy difference between these structures and the energy minimum structure was less than 18 kJ/mol, corresponding to the requirement of rotation energy for the C-C bond in ethane. It was aimed to simulate the different structures arisen from the thermal motion of the molecule. As shown in the CTS of CuSAC6N6 (Supplementary Fig. 39b-S39f), the contribution of D-A transfer transition ranged from 7.1 to 9.0%, close to that of the energy minimum structure (Fig   5c, 9.6%). These results indicated that conformational flexibility almost had no side effect on the excited state via the D-π-A electron-transfer.  Then, based on Eq. S15 and Eq. S16, Eq. S14 can be written as: Taking the indefinite integral for Eq. S17, we can obtain: where the kB is the slope of the basic reaction, it can be written as: where c1 is a constant.
According to the Lambert-Beer law, [ABTSox] is proportional to its absorbance at 417 nm. Therefore, Eq. S23 well explains the linear relationship between the absorbance of ABTSox at 417 nm and [Glu] (Fig. 6b).
where d is a constant.
Next, as the overall reaction rate would be calculated by the formation rate of the ABTSox product, according to the law of mass action, the reaction rate equations are: where f is a constant.
Here, the kG is the slope of gain reaction which increases with the enhancement of Ia.
It can be written as: We can see that kG is proportional to Ia.
Here, under light irradiation of different intensity, Eq. S25 could well explain the light intensity-dependent linear detection range and sensitivity to a diverse range of concentrations in vitro (Fig. 6b).