| Fluorescence emission spectra, excitation wavelengths 500–590 nm in 10 nm increments | Emission wavelength was excitation-wavelength dependent, with emission peaks red-shifting as the excitation wavelength increased; two prominent emission maxima were observed at 561 nm and 586 nm, corresponding to excitation wavelengths of 540 nm and 560 nm, respectively |
|---|---|
| Fluorescence emission maxima, replicate syntheses, λₑₓ 540 nm; λₑₓ 560 nm | Replicate 1: 561.02 nm; 585.07 nm. Replicate 2: 558.97 nm; 585.94 nm. Replicate 3: 561.02 nm; 586.95 nm. Replicate 4: 562.05 nm; 586.95 nm. Replicate 5: 557.94 nm; 586.95 nm. Average: 560.2 ± 1.5 nm; 586.4 ± 0.9 nm |
| Fluorescence emission maxima in aqueous media, λₑₓ 540 nm; λₑₓ 560 nm | HCl, pH 1: 560.00 nm; 588.98 nm. HCl, pH 1.5: 557.94 nm; 586.95 nm. H₂SO₄, pH 1: 560.00 nm; 586.95 nm. Phosphate buffer, pH 3, 0.3 M: 562.05 nm; 585.94 nm. Phosphate buffer, pH 3, with sucrose: 560.00 nm; 585.07 nm. Phosphate buffer, pH 3.5: 561.02 nm; 585.07 nm. Sucrose: 558.97 nm; 586.95 nm. Diethylmalonic acid–ammonium sulfate, 2:1: 560.00 nm; 586.95 nm. Malonic acid–ammonium sulfate, 2:1: 560.00 nm; 587.97 nm. Average: 560.0 ± 1.2 nm; 586.8 ± 1.3 nm |
| UV–Vis absorption spectroscopy | Three major bands were observed: 272 nm, corresponding to a π–π* transition involving aromatic C=C bonds in the carbon core; a tail at 349 nm, indicating intrinsic absorption of the n–π* transition of C=O in the carbon cores; and 540 nm, originating from surface-state transitions involving lone-pair electrons from C=O and C=N surface functional groups |
| Fluorescence quantum yield, λₑₓ 550 nm, Rhodamine B reference, water | 16.9 %; Rhodamine B quantum yield: 0.31; solvent refractive index: 1.333; i-CD fluorescence was relatively intense and comparable to p-CD fluorescence, and the i-CD quantum yield was higher than the p-CD quantum yield |
| ATR-FTIR spectroscopy | N–H/O–H: 3521 cm⁻¹; COO⁻: 1621 cm⁻¹; P=O: 1095 cm⁻¹; P–O–C: 1030 cm⁻¹; P–O–H: 949 cm⁻¹ |
| XPS survey spectrum, surface elemental concentrations | O 1s: 48.9 %; N 1s: 1 %; C 1s: 32.2 %; P 2p: 15.8 %; a small amount of indium was detected from the underlying indium-foil substrate |
| High-resolution C 1s XPS | 284.78 eV, corresponding to C–C; 286.41 eV, corresponding to C–(O, N); 289.11 eV, corresponding to –COO |
| Transmission electron microscopy | Images suggested that i-CDs generally had an aspect ratio around 1; average diameter 10.5 ± 1.8 nm, based on measurements of 525 i-CDs |
Komal Jaswal, Victoria Aderonke Adekunle, Emma C. Tackman, Miriam Arak Freedman. “Dual fluorophore system for direct measurement of acidity of aerosol particles.” ChemRxiv (). https://doi.org/10.26434/chemrxiv.15004206/v2
Calibration-performance and measurement-accuracy comparisons were not retained
| Relative-humidity phase transitions, no carbon dots, ≤2 % to 95 % RH and reverse cycling at ≤1 % RH min⁻¹, separation relative humidity; efflorescence relative humidity; deliquescence relative humidity | 73.6 ± 0.5 %; 40.8 ± 4.0 %; 79.3 ± 0.7 % |
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| Relative-humidity phase transitions, with i-CDs, ≤2 % to 95 % RH and reverse cycling at ≤1 % RH min⁻¹, separation relative humidity; efflorescence relative humidity; deliquescence relative humidity | 74.7 ± 0.5 %; 37.7 ± 3.9 %; 78.4 ± 0.5 % |
| Relative-humidity phase transitions, with i-CDs and p-CDs, ≤2 % to 95 % RH and reverse cycling at ≤1 % RH min⁻¹, separation relative humidity; efflorescence relative humidity; deliquescence relative humidity | 73.0 ± 0.7 %; 39.5 ± 3.6 %; 78.2 ± 1.0 % |
| Relative-humidity phase-transition comparison, with and without carbon dots | Differences fell within instrumental error; t-tests found no statistically significant differences at the 95 % confidence level |
| Fluorescence microscopy of i-CD distribution, λₑₓ 528–552 nm/λₑₘ 577–632 nm | At high relative humidity, i-CDs were uniformly distributed through homogeneous particles; at low relative humidity, i-CDs preferentially partitioned to the outer organic shell of phase-separated particles |
| Mean individual-droplet integrated fluorescence intensity ratio Iₚ-CDs/Iᵢ-CDs, fluorescence microscopy, λₑₓ 361–389 nm/λₑₘ 435–485 nm divided by λₑₓ 528–552 nm/λₑₘ 577–632 nm | 0.15 ± 0.01 |
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| Bulk-solution pH, pH meter | 1.53 ± 0.02 |
| Aerosol-droplet pH, ratiometric fluorescence calibration, ≥15 particles of 10–200 µm in diameter, pre-equilibrated at ≤20 % RH for 15 min then at ≥90 % and <100 % RH for ≥15 min | 1.52 ± 0.01 |
| Mean individual-droplet integrated fluorescence intensity ratio Iₚ-CDs/Iᵢ-CDs, fluorescence microscopy, λₑₓ 361–389 nm/λₑₘ 435–485 nm divided by λₑₓ 528–552 nm/λₑₘ 577–632 nm | 0.25 ± 0.04 |
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| Bulk-solution pH, pH meter | 1.69 ± 0.02 |
| Aerosol-droplet pH, ratiometric fluorescence calibration, ≥15 particles of 10–200 µm in diameter, pre-equilibrated at ≤20 % RH for 15 min then at ≥90 % and <100 % RH for ≥15 min | 1.67 ± 0.06 |
| Mean individual-droplet integrated fluorescence intensity ratio Iᵢ-CDs/Iₚ-CDs, fluorescence microscopy | 3.06 ± 0.44 |
|---|---|
| Aerosol-droplet ionic strength, ratiometric fluorescence calibration, ≥90 % relative humidity | 0.64 ± 0.58 M |