Figure 3. Selectively functionalized optical ring resonator DNA array.

(a) Electrografting of diazonium ions is used as pattering method to selectively functionalize a pair of optical ring resonator sensors within the same microfluidic chamber. (b) Each sensor is electrically isolated; thus, a different diazonium salt can be electrografted on each sensor to provide an orthogonal chemical functionality. (c) The individual resonators are then functionalized with modified DNA sequences using copper-catalysed azide–alkyne click (CuAAC) reaction, showing selective binding to the microrings functionalized with the pertinent chemical moiety. No binding is observed using an unmodified microring, as illustrated by the turquoise trace. (e) First, the azide-modified sensor is functionalized with the ssDNA_alkyne sequence. (f) Second, the alkyne-modified sensor with the ssDNA_azide sequence. Both ssDNA_alkyne and ssDNA_azide were added at a 2 μM concentration. This immobilization process was performed using potassium phosphate buffer (100 mM, pH 7). (d) The selectivity of the functionalization process is confirmed by exposing the sensor array to DNA of the reverse complement of ssDNA_azide and ssDNA_alkyne. We note that no hybridization is observed when challenging the sensor to non-complementary DNA, as illustrated by the turquoise trace. (g,h) Injection of the reverse complements of ssDNA_azide and ssDNA_alkyne, labelled as cDNA_azide and cDNA_alkyne, respectively. The reverse complements cDNA_azide and cDNA_alkyne were injected at 400 nM concentration in Tris-EDTA (TE) containing 1 M NaCl.