Hi Methee,

Nanodrop is not feasible, because miRNAs constitute only a minor fraction of total cellular RNA. You can enrich small RNAs of less than ~200 nt by differential alcohol precipitation, PEG precipitation, or other commercially available reagents (e.g. miRVana from Thermo), but the vast majority of what you get still would be highly abundant structural RNAs such as rRNA and tRNA.

Quantification is usually carried out in the field by qRT-PCR or small RNA northern blotting. Each has its own pros and cons. qRT-PCR requires only a small amount of sample and provides better sensitivity, but it easily underestimates what's called isomiR. Northern blotting requires relatively much RNA (~10-50 ug) and often suffers from poor sensitivity, although many technical advances have been made to solve this issue (EDC crosslinking, use of LNA probe, etc). However, for decently expressed miRNAs, northern blotting in conjuction wih the use of synthetic standard RNA (often called semi-quantitative northern blotting) provides much accurate information on the quantitiy of a given miRNA.

If you are interested in global changes of miRNA expression rather than expression of a few miRNAs, you can do small RNA deep sequencing.