Single-molecule experiment

A single-molecule experiment resolves a response attributable to an individual molecule, rather than only an average over many molecules. Optical detection need not resolve its constituent chemical atoms in an image: repeated photon absorption by the same molecule can produce a measurable response. One detected photon is not automatically one different molecule.

The measured channel matters. Direct photon absorption detection measures a change in transmitted light. Fluorescence detection collects photons emitted after excitation; a fluorescence excitation spectrum varies the illuminating frequency while observing that emission. Both can probe excitation, but their backgrounds and readouts differ. A labeled particle cartoon explains the interpretation; it supplies no experimental evidence by itself.

Such measurements expose differences that a population average can conceal. Suppose two groups of molecules switch between different internal quantum states. Their average response could vary smoothly even though each molecule changes in discrete steps. Resolving an individual response tests that interpretation directly, subject to the measurement's time resolution, sensitivity, and particle assignment. The following experiment makes that assignment through spectral selection and controlled perturbations.

Measure the force and extension of an individual molecular tether under controlled displacement or load. A calibrated cantilever or optical trap converts deflection to force. Synthetic polymer stretching provides a nonbiological application; the observed curve also contains instrument and attachment compliance, and its loading-rate dependence can distinguish equilibrium elasticity from activated events.

Force peaks in membrane-protein extraction can reflect steric barriers, membrane anchoring and electrostatic interactions. Changing ionic strength changes screening and possibly specific ion binding; it need not weaken every barrier in the same direction. The rising branch of an exposed chain and rupture of a remaining structural barrier are different physical processes.

A measurement protocol that holds applied force approximately constant while allowing displacement to vary. Feedback can adjust an actuator in response to a calibrated load signal; alternatively, a passive field gradient can be sufficiently uniform over the displacement range. Finite feedback bandwidth, drift and spatial force variation limit the approximation. This differs from fixing actuator position or prescribing a force ramp.

For a simple single-tether geometry, imposed displacement $Z$ divides into cantilever deflection $d$ and molecular extension $x$. Applying the displayed correction prevents attributing the instrument's stretch to the molecule. Geometric offsets, handles and substrate compliance require additional calibrated terms.

In 1989, William E. Moerner and Lothar Kador reported individual pentacene absorbers in para-terphenyl in Optical detection and spectroscopy of single molecules in a solid by W. E. Moerner and L. Kador (pp. 2535–2538). Different local environments spread photon absorption frequencies; tuning into the sparse wings selected few resonant molecules despite many molecules in the illuminated volume.

Their tunable dye laser focused through a $0.85$ numerical aperture lens onto a $100$–$200\mu \mathrm{m}$ crystal immersed in superfluid helium at $1.6\mathrm{K}$. Frequency modulation accompanied either a $5\mathrm{kHz}$ electric field between coated sapphire disks, detected at $10\mathrm{kHz}$, or a $2\mathrm{MHz}$ quartz shear transducer. An avalanche photodiode measured transmitted-light photon absorption, not fluorescence. Double modulation suppressed residual laser amplitude background.^[ref]

Repeated scans gave the predicted modulated line shape. Blank crystals and off-resonance scans gave background; changing modulation frequency expanded and contracted the feature as predicted. Agreement between both perturbation methods supported the individual-absorber inference. The authors acknowledged out-of-focus molecules and the unlikely alternative of multiple absorbers with exactly coincident frequencies.^[ref]

This is evidence for a selected absorber in a prepared, known chemical substance and host. It does not establish arbitrary unknown molecule identity from one spectral feature, provide an atom-resolved photograph, or count every molecule in the crystal. A measurement sensitive to only a narrow excitation range cannot detect a molecule whose transition lies outside that range. This distinction separates the total particle inventory from the number responding to a particular probe.