The most convenient way is to use "cysteine" as capping agent.
Here goes one if you would like to take it biocompatible !
http://iopscience.iop.org/2043-6262/5/2/025017/pdf/2043-6262_5_2_025017.pdf
Positively charged silver nanoparticles and their effect on Surface-
enhanced Raman scattering of dye-labelled oligonucleotides
Danny van Lierop, Željka Krpeti, Luca Guerrini, Iain A. Larmour, Jennifer A. Dougan, Karen Faulds and Duncan Graham*
Supplementary information
Electronic Supplementary Material (ESI) for Chemical Communications
This journal is © The Royal Society of Chemistry 2012
according to the above:
Glassware preparation for positively charged nanoparticles preparation and storage
To prevent attachment of positively charged nanoparticles to glass surfaces, vials used for particle preparation and storage were coated with Polyethyleneimine (PEI).
1Briefly, prior to use all glassware was cleaned with chromic acid overnight and extensively rinsed with milli-Q water and dried in N2 flow. The glassware was then filled with 0.2% w/w PEI aqueous solution and incubated for 2h. Vials were
then rinsed once with milli-Q water and dried with N2 flow.
Preparation of positively charged silver nanoparticles (Ag-positive)
To 10 mL aqueous AgNO3(1 mM) 0.1 M spermine hydrochloride was added (5
μL) and the mixture was degassed for 30 min under N2 flow, protected from light. Subsequently, und er vigorous stirring, 25 μL of aqueous NaBH4 (0.1 M) was quickly added to the solution and stirred for 20 min. Particle formation
was immediately observed as the colour of the dispersion turned bright yellow. Formation of the particles was verified by UV-visible spectroscopy observing a typical plasmon resonance peak in the 390-400 nm wavelength range.
Following this preparation method, Silver nanoparticles (AgNPs) stabilised with branched PEI (bPEI), spermidine, and ethylenedioxydiethylamine were prepared.