An Electrochemical Nitrite Sensor Based on Two-Dimensional Conductive Phthalocyanine-Based Metal-Organic Frameworks

Abstract

Nitrite (NO2 -) is a common environmental contaminant that is appeared in the water, soil and other environments, also served as a kind of preservative for the food industry. Nitrite-rich contaminants caused terrible impacts on the ecological environment and public health due to unreasonable utilization/treatment of nitrite in the field of farming, food industry, and environmental protection. Therefore, it is of great importance for the accurate determination of nitrite in the drinking water or pickle foods. Moreover, the World Health Organization (WHO) has established a maximum limit of nitrite dosage of 65.2 μM (3 mg L-1) in drinking water. So, the determination strategy with highly sensitive, selectively and rapid response toward nitrite is imperative. Capillary electrophoresis, spectrophotometry, and ion chromatography, etc. are useful with high sensitivity, but time-cost, more operation skills are required toward the above analytical methods. Over the above approaches, the electrochemical determination has been widely developed owing to its extra merits including real-time, low-cost, feasibility.Metal-organic frameworks (MOFs) were constructed by assembling transition metal ions and organic linkers through coordination reactions. MOFs were firstly utilized for gas adsorption and storage application due to their porous structure and large surface area. With the exploration of MOFs in the field of electrocatalysis, researchers found MOFs exposed more potential active sites on their larger surface, promoting easily the contact with target molecules, which further improve the electrocatalytic performance of MOFs, made MOFs be perfect candidates for sensing. However, great challenges remain for conventional MOFs due to their poor conductive/electronic properties, it is dramatically limited the usage of MOFs in the electrochemical applications. To remove the above challenges, several strategies were put forward, such as i) pyrolysis of MOFs, the carbonized MOFs possessed metal-doped or multi-atoms doped porous carbon, enhancing their electrocatalytic activity; ii) preparation of MOF-based hybrids, conductive supports (carbon nanotube, graphene, metal foams, etc.) were introduced for promoting their electrical conductivity; iii) synthesis of conductive MOFs, the novel conductive MOFs can improve electron transfer capacity directly without pre-treatments. However, well-defined molecular active sites on MOFs are decomposed after the high-temperature process. Also, the second method can promote their electrocatalytic activity to some extent, but it may reduce the inherent advantages of MOFs as well as decrease the surface area and reduce the accessible active sites. The later have more advantages over the other strategies, owing to the development of conductive MOFs which can solve these challenges fundamentally and avoid the former approaches’ negative effects.26Two-dimensional (2D) conductive MOFs represent an emerging class of nanomaterials, presenting their exceptional 2D characteristic, enhanced the ability of electron transfer and the high efficiency of active sites, except the intrinsic merits of conventional MOFs. Such 2D conductive MOFs offer a perfect platform for the study of the mechanism of electroanalysis, which is helpful for the enhanced sensing performance of MOFs.In this study, nickel phthalocyanine (NiPc) was selected as an organic linker to assemble 2D NiPc-MOF. Three main reasons arise from using this linker for synthesizing 2D MOF: i) metal active sites are atomically dispersed on metallophthalocyanines theoretically; ii) NiPc-MOFs extended in two-dimension with fully in-plane π delocalization and weak out-of-plane π–π stacking, further promoting electron transfer between electrocatalysts and analytes; iii) the larger surface area of 2D NiPc-MOF, the easier absorbed on the electrode, keeping its electrochemical stability, then achieving excellent sensitivity. Benefiting from its excellent conductivity and large surface area, 2D NiPc-MOF nanosheets present excellent electrocatalytic activity for nitrite sensing with an ultra-wide linear concentration from 0.01 mM to 11500 mM and a low detection limit of 2.3 μM, which is better than most reported electrochemical nitrite sensors. Significantly, this work reported the synthesis of 2D conductive NiPc-MOF and developed it as an electrochemical biosensor for non-enzymatic nitrite determination for the first time. Figure 1