Near-infrared emission TMB dots as a colorimetric and fluorescent nanoswitch for the reversible recognition of iron ion and cysteine and their logic gate applications

3,3′,5,5′-Tetramethylbenzidine (TMB) is one of the traditional enzyme substrates that has been widely used in the fields of artificial enzyme mimics and natural enzymes. However, the development of TMB-based probes for fluorescence and colorimetric dual models remains a great challenge. Herein, we developed a facile method to fabricate a colorimetric and fluorescent nanoswitch based on near-infrared fluorescent TMB dots (TMBDs) for the reversible recognition (CFRR) of Fe3+ and Cys. The proposed CFRR nanoswitch was based on an oxidation–reduction mechanism by which the TMBDs are oxidized by Fe3+ to form ensembles and Cys reduces the ensembles to TMBDs. The CFRR nanoswitch exhibited high sensitivity for the colorimetric and fluorescent recognition of Fe3+ with limits of detection (LODs) of 0.17 μM and 7.5 μM and of Cys with LODs of 0.1 μM and 1.5 μM, respectively. The response speed was very fast, within 5 min. Moreover, two output signals, namely near-infrared fluorescent and colorimetric, were exhibited in this CFRR sensor; the recognition was reversible and retained good stability after three cycles. It is noteworthy to mention that the CFRR nanoswitch performed successfully in urine, and molecular logic gate systems were readily established. Therefore, these investigations will shed light on the development of multi-mode output signals for nanoprobes.