Emission Wavelength Switchable Carbon Dots Combined with Biomimetic Inorganic Nanozymes for a Two-Photon Fluorescence Immunoassay

In this work, o-phenylenediamine is utilized as a precursor to synthesize the fluorescent emission wavelength switchable carbon dots (o-CDs). Our investigation reveals that ferrous ions (Fe2+) can effectively induce fluorescence quenching of o-CDs by chelation and aggregation. After the addition of hydrogen peroxide (H2O2), the fluorescence of o-CDs recovers and the fluorescent color changes from yellow to green. As far as we know, o-CDs are the first reported CDs with switchable fluorescence emission wavelength. In order to fabricate an enzyme-free immunosensor, an amino-functionalized dendritic mesoporous silica nanoparticle (DMSN)–gold nanoparticle (Au NP) nanostructure was fabricated as a glucose oxidase mimetic nanoenzyme by in situ coating of the Au NPs on the surface of the DMSNs. Then, the functionalized DMSN-Au NPs were modified on the detection antibody and hydrolyzed with glucose to produce H2O2. This immune induced recognition strategy combines with the o-CDs+Fe2+ signal generation system to achieve specific and sensitive detection of the target. The replacement of glucose oxidase by DMSN-Au NPs not only reduces the cost but also provides significantly amplified signals due to DMSNs haing a high specific surface area. We show the detection of carcinoembryonic antigen (CEA) as an example target to evaluate the analytical figure of merits of the proposed strategy. Under the optimal conditions, two-photon-based o-CDs displayed excellent performances for CEA and the limit of detection as low as 74.5 pg/mL with a linear range from 0.1 to 80 ng/mL. The proposed fluorescent immunosensor provides an optional and potential scheme for low cost, high sensitivity, and versatile discovery of clinical biomarkers.