Comparison of signals for varied aldehyde substrates in phosphate buffer, showing selective signaling with alkyl aldehydes. (a) Hydrazone exchange with different acceptor carbonyl substrates. Conditions: 300 mM phosphate buffer (pH 7), 500 nM fluorescein DarkZone dye, 5 mM catalyst 3, and 2 mM carbonyl substrates. Aldehydes: glyoxylic acid (7), 2-methylbutyraldehyde (8), l-rhamnose (9), pyridine-2-carboxaldehyde (10), phthalaldehyde (11), pyridoxal (12), and 4-nitrobenzaldehyde (13).