Dual-emitting quantum dot nanohybrid for imaging of latent fingerprints: simultaneous identification of individuals and traffic light-type visualization of TNT

A nanohybrid was employed for fingerprint imaging that was capable of simultaneous identification of individuals and TNT visualization in a “traffic-light” manner.

The pH of the mixed solution was adjusted to ~11 with 2.0 M NaOH. Freshly prepared Na 2 S (0.2 mL, 0.4 M) was quickly injected with stirring for 2 min. Then, the mixture solution was refluxed at 60 ºC for 2 min. Afterwards, Zn(NO 3 ) 2 (0.08 mL, 0.2 M) was injected into the reaction solution, and refluxed at 100 ºC for another 2 min.
The obtained green (ZnCdS) QDs were precipitated with ethanol, and separated by centrifugation. The resultant QDs were directly dissolved in water for further use.

Preparation of Cu-doped ZnCdS QDs
For Cu-doped ZnCdS QDs, Zn(NO 3 ) 2 (0.05 mL, 0.2 M), Cd(NO 3 ) 2 (0.8 mL, 0.4 M), Cu(Ac) 2 (0.082 mL, 0.02 M), and MPA (0.056 mL) were combined with 40 mL ultrapure water in a four-neck round-bottomed flask. The pH of the mixed solution was adjusted to ~11 with 2.0 M NaOH. Freshly prepared Na 2 S (0.8 mL, 0.4 M) was quickly injected with stirring for 10 min. Then, the mixture solution was refluxed at 100 ºC for 2 h. Afterwards, Zn(NO 3 ) 2 (0.08 mL, 0.2 M) was injected into the reaction solution, and refluxed at 100 ºC for another 10 min. The obtained MPA-capped Red (Cu-doped ZnCdS) QDs were precipitated with ethanol, and separated by centrifugation. The resultant QDs were directly dissolved in water for further use.

Preparation of red fluorescent QD-doped silica nanoparticles 2
The red fluorescent QDs incorporated silica nanoparticles were synthesized by a modified Stöber method. [S1] Briefly, ethanol (20 mL), MPS (10 µL), red-emitting Cu-ZnCdS QDs (10 mL, pH 11) were mixed under stirring in a 100 mL three-necked flask for 6 h. Then, 0.2 mL of TEOS and 0.2 mL of ammonium hydroxide were added, and the mixture was stirred for 12 h. To modify the silica surface with amino groups, 10 µL of APTS was added to the above solution and stirred for another 12 h. After the reaction, the as-prepared nanoparticles were washed with ethanol and ultrapure water for several times to remove the un-reacted chemicals.

Synthesis of dual-emitting red QDs@silica@green QDs nanohybrid
4 mL green QDs solution and 4 mL red QD-doped silica nanoparticles were dispersed in 4 mL ultrapure water, and then 2 mL of EDC/NHS (2 mg/mL) was added. The final mixture was stirred for 2 h at room temperature in the dark. The resulting nanohybrid of red QDs@silica@green QDs were isolated by centrifugation and washed with ultrapure water to remove the excess QDs and chemicals. The nanohybrid was further modified in the mixed solution of 0.5 mg polyallylamine, 1 mL EDC/NHS (2 mg/mL) and 8 mL ultrapure water by shaking for 0.5 h. The obtained nanohybrid was washed with ultrapure water and re-dispersed in 5 mL ultrapure water.         Figure S13. Imaging of LFPs with or without TNT on glass and paper substrates with the green/red QDs nanohybrid. Note, the image color of LFP on these substrates is different from that on aluminum foil strips (in the text), because these substrates have background fluorescence. All fluorescent images were excited with a 365 nm UV lamp.