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Fluorescent gold nanoclusters (Au NCs) with excellent one-photon and multiphoton properties have been demonstrated as promising candidates in many application fields. However, small multiphoton absorption (MPA) cross sections and weak multiphoton excitation (MPE) fluorescence impede their practical applications under near-infrared (NIR) excitation for biological imaging. Here, we report the regulated one-photon and multiphoton properties and mechanisms of arginine-stabilized 6-aza-2-thiothymine Au NCs (Arg/ATT-Au NCs) and the applications for MPE fluorescence imaging. The introduction of arginine into the capping layer of ATT-Au NCs significantly modifies the electronic structure, the absorption cross sections, and the relaxation dynamics of the lowest excited state, drastically reducing the nonradiative relaxation, suppressing the blinking, and greatly enhancing the fluorescence. Besides the improved one-photon properties, Arg/ATT-Au NCs demonstrate remarkable MPE fluorescence with a large MPA cross section. The two-photon (λ

Blood coagulation factor VIII (FVIII) can bind calcium ions and ion-protein interactions appear central importance for both their structure and function in coagulation cascade. However, the mechanism and details of how calcium dependent structure change of proteins to fulfill their function remain to be fully defined. In this work, PeakForce Quantitative Nanomechanics (PF-QNM) mode atomic force microscopy (AFM) was used to map the topography and mechanical properties of FVIII with single protein resolution under different calcium concentrations. The obtained nanomechanical spectroscopy showed that calcium ions play dual roles in the chain association and structural flexibility of FVIII. Low concentration of calcium ions prefer to bind isolated chains and increase their mechanical properties, whereas they link the heavy and light chains to keep the protein re-association under higher ions concentration. Our results provide a novel insight into the mechanistic details of the metal ions on the stability and function of blood clotting proteins.