A. Doron et al., PHOTOCHEMICALLY-CONTROLLED, CHEMICALLY-CONTROLLED, AND PH-CONTROLLED ELECTROCHEMISTRY AT FUNCTIONALIZED SPIROPYRAN MONOLAYER ELECTRODES, Langmuir, 13(6), 1997, pp. 1783-1790
A photoisomerizable nitrospiropyran monolayer assembled on a Au electr
ode provides a functionalized interface for the photochemical, pH, and
thermal control of electrochemical processes of charged electroactive
redox probes. (Mercaptobutyl)nitrospiropyran 1 was assembled as a mon
olayer on a Au electrode. The monolayer exhibits reversible photoisome
rizable features, and illumination of the nitrospiropyran monolayer, S
P state, 320 nm < lambda < 350 nm, yields at pH = 7.0 the protonated n
itromerocyanine monolayer state, MRH(+) state. Further irradiation of
the MRH(+) monolayer, lambda > 495 nm, regenerates the SP state of the
monolayer. The light-induced transformation of the monolayer between
a neutral and a positively-charged interface allows the control of the
electron transfer processes at the electrode interface. Electrooxidat
ion of the negatively-charged (3,4-dihydroxyphenyl)acetic acid, DHPAA,
is enhanced at the MRH(+) monolayer electrode as compared to the SP-f
unctionalized monolayer electrode. Electrooxidation of the positively-
charged 3-hydroxytyramine (dopamine), DOPA, is inhibited at the MRH(+)
monolayer electrode as compared to its oxidation by the SP monolayer
electrode. The control of the electrochemical oxidation of DHPAA and D
OPA at the photoisomerizable monolayer electrode is attributed to the
electrostatic interactions of the MRH(+) monolayer electrode with the
redox-active substrates. Electrostatic attraction of DHPAA and repulsi
on of DOPA by the MRH(+) monolayer results in enhancement or inhibitio
n of the electrochemical processes, respectively. By reversible isomer
ization of the monolayer between the SP and MRH(+) states, cyclic ampe
rometric transduction of the optical signals recorded by the monolayer
is accomplished. In the presence of a mixture of oppositely-charged r
edox substrates, i.e. DHPAA and dimethylbutylammonio)ethyl]amino]-1,4-
benzoquinone (3) or pyrroloquinoline quinone, PQQ (4) and 3, photostim
ulated selective electrochemistry is accomplished in the presence of t
he photoisomerizable monolayer electrode. The transformation of the pr
otonated nitromerocyanine monolayer, MRH(+) state, generated at pH = 7
.0, to the zwitterionic nitromerocyanine configuration, MR(+/-) state
at higher pH, allows the pH-controlled electrooxidation of DHPAA and D
OPA at the monolayer electrode. Similarly, thermal isomerization of th
e SP monolayer electrode, pH = 7.0, 60 degrees C, yields the MRH(+) mo
nolayer electrode. These thermochromic features of the monolayer are e
mployed to respectively activate or deactivate the electrooxidation of
DHPAA or DOPA at the functionalized electrode. By cyclic thermal isom
erization of the SP monolayer to the MRH(+) monolayer followed by phot
ochemical isomerization of the MRH(+) monolayer followed by photochemi
cal isomerization of the MRH(+) monolayer to the SP state, lambda > 49
5 nm, the thermochromic and photochromic features of the monolayer are
amperometrically transduced via the oxidation of DHPAA and DOPA, resp
ectively. Electrochemical oxidation of DHPAA and DOPA is further accom
plished by the application of a dinitrospiropyran monolayer (2) electr
ode in the presence of the dinitrophenyl antibody, DNP-Ab. (Mercaptobu
tyl)dinitrospiropyran 2 was assembled as a monolayer on a Au electrode
. The dinitrospiropyran monolayer, SP state, exhibits antigen features
for the DNP-Ab, where the protonated dinitromerocyanine monolayer, MR
H(+) state, lacks antigen features for the DNP-Ab. Association of the
DNP-Ab to the SP monolayer electrode blocks the electrooxidation of DH
PAA or DOPA. Photochemical isomerization of the SP monolayer to the MR
H(+) state, 320 nm < lambda < 350 nm, results in the release of DNP-Ab
and the activation of the electrooxidation of DHPAA and DOPA. By the
reversible photoisomerization of the monolayer between the SP and MRH(
+) states in the presence of DNP-Ab, cyclic amperometric transduction
of the optical signals recorded by the monolayer is accomplished.