X-Ray photoelectron spectroscopy (XPS) analysis
The surface analysis studies were performed in a UHV chamber (P<10-9 mbar)
equipped with a SPECS LHS-10 hemispherical electron analyzer. The XPS
measurements were carried out at room temperature by using unmonochromatized
AlKa radiation under conditions optimized for maximum signal (constant ΔΕ mode
with pass energy of 36 eV, giving a full width at half maximum (FWHM) of 0.9 eV
for the Au 4f7/2 peak). The analyzed area was ellipsoid with dimensions of 2.5 x 4.5
mm2. The XPS core level spectra were analyzed using a fitting routine, which allows
the decomposition of each spectrum into individual mixed Gaussian-Lorentzian
components after a Shirley background subtraction.
Wide Scans were recorded for all samples, while the core level peaks that were
recorded in detail were: O1s, C1s, N1s and Na1s. Errors in quantitative data were
found to be in the range of ~10%, (peak areas), while the accuracy for BEs
assignments was ~0.1 eV. The samples were in liquid form; a few drops were applied
on a polycrystalline Si specimen with dimensions of 1x1cm2 (drop casting method)
and were left to dry for many hours at 400C.
Table SI 1. Assignment of FT-IR frequencies of CA-dots
ATR (FT-IR) (cm-1)
Assignment
3296
O-H symmetric stretching
2935
C-H stretching of (-CH2)
2864
C-H stretching of (-CH2)
1695
C=O carbonyl stretch
1635
C=O (amide I)
1542
N-H bending or C-N stretching
(amide II)
1405
COO- stretching
1266
C-OH
1168
C-O-C symmetric stretching
877
NH2 bending
Table SI 2. Assignment of FT-IR frequencies of CU-dots
ATR (FT-IR) (cm-1)
Assignment
3190
N-H symmetric stretching
3040
O-H symmetric stretching
2875
C-H stretching of (-CH2)
2776
C-H stretching of (-CH2)
1774
imide (C=O)
1711
C=O carbonyl stretch
1655
C=O (amide I)
1559
N-H bending or C-N stretching
(amide II)
1391
COO- stretching
1354
C-H stretching
1280
C-OH
1190
C-O-C symmetric stretching
890
NH2 bending
752
C-N-H stretching
Quantum Yield Measurements
The fluorescence quantum yield (Φ) of CA and CU dot solutions was measured by comparing
the integrated fluorescence emission intensity against two reference fluorophores: anthracene
(An) (λexc: 340 nm) and quinine sulphate (QS) (λexc: 348 nm) (Fig SI5). Anthracene (literature
Φ=0.27) was dissolved in ethanol (refractive index, n = 1.36). Quinine sulphate (literature
Φ=0.54) was dissolved in 0.1M H2SO4 (refractive index, n = 1.333 (Melhuish et al 1961).
CA- and CU-dots were dissolved in distilled water (n = 1.333).
The fluorescence quantum yield was calculated using the following equation:
ΦX = ΦST (GradX/GradST)(n2X/n2ST)
Subscripts ST and X denote to standard and analyte respectively.
Φ : fluorescence quantum yield
Grad : the slope of the line fitting integrated fluorescence intensity vs solution absorbance
n : refractive index of the solvent.
In order to minimize re-absorption effects, absorbance values of solutions in a 10 mm optical
path length cuvette were maintained below 0.10 at the excitation wavelengths (340, 348 nm).
Excitation and emission slit widths were set such that excitation and emission bandwidths
were 1.0 and 5.0 nm respectively when recording the fluorescence emission spectra.
Intensity (Fluorescence)
3x10
8
2x10
8
1x10
8
CA
CU
q. sulfate
anthracene
9
GradQS=4.1222x10
9
GradAn=2.56049x10
2
R =0.995
2
R =0.952
9
GradCU=1.11528x10
2
R =0.969
0
0,00
GradCA= 6.26756x108
2
R =0.985
0,02 0,04 0,06 0,08 0,10
Concentration (Abs 335nm)
Fig. SI 1 Integrated fluorescence intensity versus absorbance for CA and CU-dots, and
quinine sulfate and anthracene solutions
White light illumination
L.E.D. (exc:370 nm)
Fig. SI 2. Appearance of CA-dots in a) white and b) UV light