Sensors

What does a Jablonski diagram represent?

A diagram showing energy states (S₀, S₁, T₁) and transitions (radiative and nonradiative) after light absorption.

What are the timescales of radiative and nonradiative transitions?

Fluorescence: 10⁻⁹–10⁻⁷ s; Phosphorescence: µs–s; ISC: 10⁻⁸–10⁻⁶ s; IC: 10⁻¹²–10⁻⁸ s.

What is the difference between fluorescence and phosphorescence?

Fluorescence is spin-allowed and fast; phosphorescence is spin-forbidden and slow.

What is emission quenching?

A reduction in fluorescence intensity due to interaction with a quencher molecule.

What is dynamic quenching?

Quenching due to collisions between excited fluorophores and quenchers; reduces fluorescence lifetime.

What is static quenching?

Quenching due to ground-state complex formation; does not affect the lifetime of free fluorophores.

How can dynamic and static quenching be experimentally distinguished?

Dynamic quenching decreases lifetime; static quenching does not but may change absorption spectrum.

What is Förster energy transfer (FRET)?

Nonradiative dipole-dipole energy transfer from excited donor to nearby acceptor without light emission.

What are the conditions for FRET to occur?

Spectral overlap, 1–10 nm donor-acceptor distance, and favorable dipole orientation.

What is the Förster radius (R₀)?

Distance at which FRET efficiency is 50%; depends on spectral overlap, quantum yield, orientation, and refractive index.

Why is FRET used in sensing?

Acts as a molecular ruler, enabling detection of molecular proximity, folding, or interactions.

What is solvatochromism?

The change in a molecule’s absorption/emission spectra due to solvent polarity.

What is positive solvatochromism?

Red shift of emission/absorption in more polar solvents; excited state is stabilized more.

What is negative solvatochromism?

Blue shift in more polar solvents; ground state is stabilized more.

What are the components of a fluorescent sensor?

Fluorophore (light-emitting) and ionophore (ion-binding).

What emission changes are used in sensor design?

Intensity, wavelength shift, ratiometric changes, lifetime, anisotropy.

What is a fluoroionophore?

A molecule combining a fluorophore and ionophore to detect ions via fluorescence change.

What are key features of a good fluorescent probe?

Selective ion binding, high photostability, low background, clear signal change.

What is epi-fluorescence microscopy?

Microscopy using same objective for excitation and emission collection with filters and dichroic mirror.

Why is two-photon excitation useful in biology?

Allows deep tissue imaging, minimizes photodamage, and localizes excitation.

How does a FET-based chemical sensor work?

Analyte interacts with channel material, altering its conductivity and changing the drain current.

What are MIT polymers?

Polymers that switch between conducting and insulating states under stimuli like temperature or doping.

What reactions affect organic semiconductors?

Photooxidation, redox, hydrolysis, doping, thermal degradation, cross-linking.

How do external fields affect inorganic semiconductors?

They bend energy bands and move carriers; ions can form double layers affecting conductivity.

What are key sensor performance metrics?

Sensitivity, detection limit, response time, linearity, drift, operational lifetime.

What is a photovoltaic cell?

A device converting light to electricity via the photovoltaic effect in a semiconductor.

What are the main parameters of a solar cell?

Voc, Isc, Pmax, FF, efficiency, QE, Rs, Rsh.

What is a photodetector?

A device that converts light into an electrical signal; measured via responsivity, QE, NEP, D*.

How is a photodiode different from a photovoltaic cell?

Photodiode is for light detection (reverse bias), PV cell generates power (zero bias).

How does a thermocouple work?

Two metals form a junction where temperature difference creates a measurable voltage.

What is a bulk heterojunction?

A donor–acceptor blend in organic photovoltaics that increases interfacial area for exciton dissociation.

What is a MIP (molecularly imprinted polymer)?

Polymer formed with a template to create selective recognition sites for sensing or separation.