Mixed
micelles formed by triblock
copolymers [poly(ethylene oxide)
m
(EO
m
)–poly(propylene oxide)
n
(PO
n
)–EO
m
] with various surfactants have widespread
applications. Molecular-level understanding of the composition, interfacial
organization, and hydration of the copolymer–surfactant mixed
micelle is greatly necessary from application perspectives. Here,
we applied 8-hydroxypyrene-1,3,6-trisulfonate (HPTS) to probe the
mixed micelle of a triblock copolymer F127 (EO101–PO56–EO101) and a cationic surfactant dodecyltrimethylammonium
bromide (DTAB) at various compositions. The emission spectrum of HPTS
modulates anomalously with the variation of DTAB concentration, displaying
at least four regimes. The ratio of the emission intensities of the
two bands (protonated/deprotonated) (1) first increases steeply in
the low-concentration range (0.1–6 mM), (2) remains almost
steady at the intermediate concentration (8–20 mM), (3) decreases
at high concentration (20–80 mM), and (4) finally, remains
almost constant at a very high concentration (100–400 mM) of
DTAB. Time-resolved measurements confirm that excited-state proton
transfer dynamics varies unusually with the concentration of DTAB
in the mixed micelle; substantial retardation is observed up to ∼12
mM, but after that, the dynamics becomes somewhat faster upon further
addition. The rotational dynamics of a methoxy analogue of HPTS, 8-methoxypyrene-1,3,6-trisulfonate,
becomes slower up to ∼12 mM DTAB and after that becomes faster
at higher concentration. Moreover, dynamic light scattering measurements
showed that the size of the mixed micelle decreases sharply in the
low-concentration region (<20 mM DTAB) but decreases moderately
at high concentration. Thus, the nature of the mixed micelle is very
different at low and high concentrations of DTAB. At low concentration,
the incorporation of DTAB results in a more compact and less hydrated
mixed micelle, whereas a more hydrated and less organized assembly
is formed at high concentration of DTAB.
The demand for miniaturized point‐of‐care chemical/biochemical sensors has driven the development of field‐effect transistors (FETs) based pH sensors over the last 50 years. This paper aims to review the fabrication technologies, device structures, sensing film materials, and modeling techniques utilized for FET‐based pH sensors. We present the governing principles of potentiometric sensors, with major focus on the working principles of ion‐sensitive FETs (ISFETs). We extensively review different sensing film materials deposited by various techniques, which is critical to the sensing performance of ISFETs. The popular fabrication technologies have been presented, with special emphasis on state‐of‐the‐art silicon‐on‐insulator based technology, which can achieve high sensitivity by utilizing the dual‐gate effect. Furthermore, recent advancements in nano‐ISFETs has been elucidated. We also discuss the adoption of unmodified complementary metal‐oxide semiconductor (CMOS) ISFETs using standard CMOS processes, which has enabled the fabrication of integrated ISFET arrays, which are especially suited for ion‐imaging applications. Moreover, recent developments in extended‐gate FETs has been discussed, which have gained lot of attention due to their design flexibility and ease of fabrication, which is desirable for wearable sensing applications. In addition, recently there have been efforts to utilize nonsilicon channel materials for pH‐sensing application to obtain superior performance and various channel materials have been reviewed. Finally, we have extensively reviewed the ISFET device modeling and simulation techniques using various computer‐aided design tools, which aid in sensor design and characterization.
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