373Optical Nanosensors
molecules [such as proteins] by engulng them) or imaging of cellular reac-
tions in the cytoplasm or the nucleus with QDs necessitates sophisticated
tools. Positively charged peptide (a molecule consisting of two or more amino
acids)-transduction domains such as Tat (peptide from the cationic domain
of the HIV-1 Tat protein), polyarginine (cationic peptide that promotes trans-
fection efciency under a proper concentration), polylysine (a homopolymer
of L-lysine, an essential amino acid), and other specically designed CPPs
can be coated on to QDs for their delivery into cells. The adaptation of other
recently developed cell penetrating agents, such as streptaphage, a synthetic
ligand based on an N-alkyl derivative of ...