Stabilization of silver metal in citrate buffer: Barcoded nanowires and their bioconjugates

Nanobarcodes (NBCs) are striped metallic nanowires typically several micrometers in length and similar to 300 nm in diameter, having segments of Au and Ag along their length. NBCs are promising for multiplexed bioanalysis due to the large number of striping patterns that can be synthesized and the ease of optical read-out using simple reflectance microscopy. Since Ag metal is susceptible to oxidation, we were interested in determining the long-term stability of NBCs in aqueous buffers. We report the effect of storage in high-salt hybridization buffer for both undisturbed and continuously vortexed NBC samples. We find that wires stored in hybridization buffer for longer than 2 weeks begin to show significant degradation of Ag segments. When agitated with continuous vortexing, the Ag oxidation progressed more rapidly, rendering the NBCs stored in hybridization buffer unidentifiable in less than I week. Addition of 40 mM citrate as a mild reducing agent increased Ag stability by 17 weeks over those stored in hybridization buffer. NBCs subjected to continuous vortexing in 40 mM citrate buffer retained Ag segment stability for longer than 2 weeks. Derivatization of the wires with biomolecules, such as are used in bioassays, affords some additional protection against Ag degradation. We find that wires coated with rhodamine-tagged DNA oligonucleotides attached via Neutravidin-biotin chemistry are stable for 12 days in hybridization buffer and for at least 63 days when 40 mM citrate is added. Silver deterioration in these experiments was coupled to loss of fluorescence from the labeled DNA, as well as wire breakage.