The FRET efficiency was strongly dependent on the following
conditions: (1) The emission band of the donor should
overlap with the absorption band of the acceptor. (2) The
distance between both species should be in close proximity
(less than 10 nm).62 One of the advantages of C-dots as electron
donor was that their emission wavelength can be manipulated
by choosing different excitation wavelengths thus
ensuring adaptability to any kinds of energy acceptors. For
efficient FRET, the excitation wavelength of 350 nm was
selected for excitation of C-dots (the energy donor) because
the fluorescence at 350 nm excitation overlapped with the absorption
band of R6G (the energy acceptor) to ensure the FRET
process (Figure 1). Therefore, feasible energy transfer from
C-dots to the rhodamine moiety was theoretically established.
R6G-COOH was synthesized in two steps using R6G as the
starting material (Scheme S1). Intermediate R6G-NH2 was
prepared in a reaction between R6G and 1,2-ethylenediamine
according to the previous report.58 R6G-NH2 was reacted with
succinic acid to obtain R6G-COOH. The chemical structures
of R6G-NH2 and R6G-COOH were confirmed by 1
H NMR
and FT-IR. R6G-COOH was designed to become immobilized
on the surface of C-dots through the reaction between the
carboxylic acid group in R6G-COOH and the amino group
in C-dots to form an amide linkage. Finally, rhodamine
spirolactam was successfully incorporated in C-dots via the
conventional DCC/DMAP method to obtain C-dots-R6G
(Scheme 1).