5%, Sigma-Aldrich, St. Louis, MO, USA), 4-nm Qdot® 525 ITK™ amino (PEG) quantum dots (8-μM solution 5-Fluoracil in 50 mM borate, pH 9.0, Invitrogen, Life Technologies, Carlsbad, CA, USA), 16-mercaptohexadecanoid acid
(90%, HS(CH2)15COOH, Aldrich), and deionized (DI) water. N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC; Sigma-Aldrich), N-hydroxysulfosuccinimide sodium salt (sulfo-NHS; 97%, Aldrich), and phosphate-buffered saline (PBS; pH 7.4, 10×, Invitrogen) were used for bioconjugation. Instruments This study used a NanoWizard® AFM (JPK Instrument, Berlin, Germany), MFP-3D-BIOTM AFM (ASYLUM RESEARCH, Goleta, CA, USA), HITACHI S-4800 field emission scanning electron microscope (FE-SEM; Chiyoda-ku, Japan), JEOL 2000 V UHV-TEM H 89 concentration (Akishima-shi, Japan), MicroTime 200 fluorescence lifetime systems with inverse time-resolved fluorescence microscope (PicoQuant, Berlin, Germany), and ULVAC RFS-200S RF Sputter System (Saito, Japan). We also employed 24 mm × 50 mm glass coverslips, a Lambda microliter pipette, and spin coating machine TR15 (Top Tech Machines Co., Ltd., Taichung, Taiwan) for the preparation of samples. Standard
silicon polygon-pyramidal tips (Pointprobe® NCH probes, tip radius of curvature <12 nm, resistivity 0.01 ~ 0.025 Ω cm, NanoWorld, Neuchâtel, Switzerland) supported by a cantilever with a spring constant k ~ 42 N/m were used for the attachment of Au-NPs. For Au-NP support during the attachment process, we used conductive n-type polished Si (100) wafers (resistivity 0.008 ~ 0.022 Ω cm), purchased from Swiftek Corp. (Hsinchu, Taiwan). An oscilloscope (LeCroy waveRunner 64Xi, 600 MHz, 10 GS/s, Teledyne LeCroy GmbH, Heidelberg, Germany) was used to measure the electric Ribonucleotide reductase potential. A waveform generator (WW2572A, 250 MS/s, Tabor Electronics, Tel Hanan, Israel) was employed to produce signals on demand. Sample preparation (Au-NPs) A diluted Au-NP solution was prepared by combining the initial Au-NP solution and ethanol at a volume ratio of 1:1,000. Au-NPs were then spread as a monolayer on
an n-type silicon wafer by spin-coating. The roughness of the silicon wafer surface had to be sufficiently low (on the order of 100 pm) to ensure that Au-NPs could be imaged using the NanoWizard® AFM. Sample preparation (QDs) A diluted solution of QDs was prepared by combining the initial Qdot® 525 solution with DI water at a volume ratio of 1:10,000. The diluted QD solution was then spread as a monolayer on a glass coverslip by spin-coating. The prepared sample was loaded into a fluorescence microscope. Homemade glass/Au film (65 nm) Half of the 24 mm × 50 mm glass coverslip area was exposed to a sputter source (Au) at a sputter rate of 3 Å/s. AFM images reveal an Au film thickness of 65 nm (see Additional file 1). Confocal examination To provide excitation, a picosecond diode laser (λ = 532 nm) was focused on a diffraction-limited spot using an oil-immersion objective lens (N.A. = 1.