Concentration-dependent multicolor emission is an unusual yet appealing photoluminescence property of various carbonaceous nanomaterials with interesting potential applications. While carbon dots (CDs) are no exception, the predictability and tuning of the microenvironment of CD to make it suitable for displaying concentration-dependent multicolor emission is far from adequately understood. Through the novel synthesis of bromine-doped CDs (Br-CDs) via controlled hydrothermal pyrolysis, we demonstrate the capacity of the same Br-CD to emit intense red (650 nm) as well as blue fluorescence (410 nm) including intermittent colors as a function of concentration and excitation wavelength. The concentration-dependent morphological transition of the Br-CDs was ascertained using electron microscopy shedding light on their optical evolution in response to concentration changes. The phenomenon is validated as being driven by unique rearrangement and surface functionality modulation, which is essentially linked to the concentration of CD in an ensemble. Notably, the synthesized Br-CDs displayed excellent enzyme-mimicking abilities where oxidase-like activity was assessed using a tetramethylbenzidine (TMB) substrate under visible light (LED, 23W), and peroxidase-like activity was evaluated with TMB and H2O2 over a wide range of pH and temperature. The visible-light-triggered generation of Reactive Oxygen Species (ROS) by Br-CDs proved to be an effective antibacterial agent demonstrating a significant eradication rate against both Gram-positive and Gramnegative bacteria. A captivating and unusual photophysical phenomenon is exhibited by Br-CD, showcasing their versatile applications in nanozymes and antibacterial interventions where emission color directly links to the activity eliminating the necessity of multiple titrations to determine concentration/units/dosage.

Bromine-doped carbon dot: concentration-dependent multicolor emission, nanozyme activity, and visible-light-induced photodynamic bacterial inactivation
    Abstract
        Graphical abstract
    1 Introduction
    2 Results and discussion
        2.1 Optical absorption and luminescence of Br-CD
        2.2 Structure, morphology, and functional group characterization of Br-CD
        2.3 Dual-response nanozyme activity of Br-CD
        2.4 Concentration-dependent photodynamic antibacterial activity of Br-CD
    3 Conclusions
    4 Experimental section
        4.1 Materials
        4.2 Preparation of Br-CD
        4.3 Study the dual-response nanozyme activity of Br-CD
        4.4 Kinetic and mechanism analysis
        4.5 Stability test of Br-CD as nanozymes
        4.6 Antibacterial activity
    Acknowledgements 
    References

• Maansi Aggarwal(Department of Chemistry, Indian Institute of Technology Patna, Patna 801103, Bihar, India)
• Prolay Das(Department of Chemistry, Indian Institute of Technology Patna, Patna 801103, Bihar, India) Corresponding author
• Suman Nayak(Department of Chemistry, Indian Institute of Technology Patna, Patna 801103, Bihar, India, Materials Research Centre, Division of Chemical Sciences, Indian Institute of Science, C. V. Raman Road, Bangalore 560012, Karnataka, India)