Utilizing polyethyleneimine-capped silver nanoclusters as a new fluorescence probe for Sudan I–IV sensing in ethanol based on fluorescence resonance energy transfer

In this work, fluorescence resonance energy transfer (FRET) between Sudan I–IV and polyethyleneimine-capped silver nanoclusters (PEI-AgNCs), which are smaller than the critical size for electronic energy quantization (∼2   nm), is investigated and subsequently utilized for Sudan I–IV sensing in ethanol. The PEI-AgNCs exhibit strong blue fluorescence when excited at 375   nm, and the fluorescence would be greatly enhanced when the nanoclusters are dissolved in given organic solvents, especially in n-propanol and ethanol. The characteristic emission peak of Ag nanoclusters is centered at 450   nm in ethanol. Upon addition of Sudan dyes, the fluorescence of PEI-AgNCs can be efficiently quenched. The fluorescence quenching efficiency was proportional to the concentration of Sudan dyes. Thus, precise quantification of Sudan I–IV can be made. Under optimum conditions, this fluorescence method shows a wide linear range of 0.4–25   μM Sudan I, 0.4–30   μM Sudan II, 0.2–20   μM Sudan III, and 0.2–25   μM Sudan IV, with the corresponding detection limits (3 σ /slope) of 11.0, 12.2, 8.0, and 7.6   nM, respectively. Using commercial chilli powder as a model sample, detection of Sudan I–IV could be made with satisfactory results. Furthermore, the sensing mechanism for detection of Sudan dyes is discussed.