Figure (b) shows the electrochemical reduction of the diazonium group at the PPBD film surface. Reduction of the diazo group generates aryl radicals in the film, which irreversibly crosslink with each other. Because of the broad, irreversible peak shape, direct electrochemical surface coverage is hard to measure for this process but a direct estimate from integration of the peak gives 8.6x10 -11 mol/cm 2 , reasonably consistent with the ferrocene surface coverage obtained from a film deposited under the same conditions. (Figure ) The infrared spectrum of the PPBD film, Figure , shows the distinctive stretching mode of a diazonium nitrogen-nitrogen triple bond stretch at 2250 cm -1 . The proposed polyphenolic nature of the film deposited from water is consistent with the large O-H stretching peak near 3400 cm -1 and likely C-O stretch at 1090 cm -1 . A weak peak near 1429 cm -1 is likely due to azobenzene linkages in the film, which are IR-forbidden in unsubstituted azobenzenes but can be seen in the IR spectrum when asymmetrically substituted. This absorption is blue-shifted relative to the IRforbidden mode of azobenzene (1576 cm -1 ) due to the effect of the electron-donating hydroxy group. Figure shows IR of a fresh film and the same film aged 3h in the laboratory environment. Roughly 50-80% of the intensity of the diazonium triple bond stretching peak has vanished after this time, but active diazonium groups are still present in the film. This result is reasonably consistent with the report of Downard et al. that approximately 50% of surface-bound diazonium groups on a 5 nm film produced by a two-step process on gold remain after 1 h. Both Downard et al. and Tour et al. have found that in highly conjugated molecules, spontaneous reduction of the diazonium groups by the substrate can occur instantly; this effect does not appear to be dominant on any of the substrates we examine in this work. On the other hand, irradiation of the PPBD film for 10 min with a common 254 nm UV lamp removes all traces of the N 2+ triple bond stretching peak, consistent with known reactions of other diazonium-bearing polymers. A variety of nucleophiles can readily react with the PPBD film to become attached to the surface. Ferrocene reacts in an interesting Gomberg-Bachman-Hey arylation with elimination of nitrogen to form a new C-C bond. (Figure ) This reaction has been used in several reports to probe surface density of diazonium groups. Other aromatics with electrondonating substituents react in an azo coupling, giving azobenzene derivatives on the surface. Strong inorganic nucleophiles react in a free-radical process similar to the wellknown Sandmeyer reaction to displace diazonium group. We reacted PPBD films with ferrocene, β-naphthol, hydroquinone, catechol, and potassium iodide to produce functionalized films in each case. In the case of ferrocene, the iron atom can be seen in XPS, (Figure ) while the carbon spectrum shows C=C and C-O peaks but does not show any evidence of a low-bindingenergy C-Au peak. (Figure ) Iodine is incorporated in the film after reaction with KI, (Figure ) and the dramatic two-electron proton-coupled redox reaction of immobilized hydroquinone can be observed by CV after coating of a commercial screenprinted conductive carbon ink electrode with PPBD in water, followed by reaction with aqueous hydroquinone. (Figure ) It is not surprising that PPBD can modify a carbon-based material, as many examples of diazonium-based functionalization techniques exist for conductive carbons. However, it is particularly useful that PPBD in water can be used to modify these electrodes, which are quickly destroyed by contact with organic solvents. Similarly to applications of diazo resins, this PPBD-based coating technique could be used to immobilize an enzyme on a disposable electrode for a quick and easy route to an integrated electrochemical biosensor.