Virucidal activity of silver nanoparticles against M and T tropic HIV-1. Serial two-fold dilutions of silver nanoparticles were added to 105 TCID50 of HIV-1Bal (A) and HIV-1IIIB (B) cell-free virus with a 0.2-0.5 m.o.i. After incubation for 5 min and 60 min, the mixtures were centrifuged three times at 10,000 rpm, the supernatant fluids removed, and the pellets washed three times. The final pellets were placed into 96-well plates with HeLa-CD4-LTR-β-gal cells. Assessment of HIV-1 infection was made with a luciferase-based assay. The percentage of residual infectivity after silver nanoparticle treatment was calculated with respect to the positive control of untreated virus. The assay was performed in triplicate; the data points represent the mean, and the solid lines are nonlinear regression curves done with SigmaPlot 10.0 software.