Spectroscopy: UV-Vis, Photoluminescence, and Applications (Comprehensive Lecture Notes)

Course logistics and context

• Access to GraphPad in the library; some students had trouble finding computers; a discussion board lists locations.
• Schedule note: two-hour session today, but spectroscopy part will finish a bit earlier; next week to cover electrophoresis and hoped-for access to online modules/tools.
• Plan: later modules include online quizzes or practice tools once access is granted.

What is spectroscopy and why do we care?

• Spectroscopy comes from Latin/Greek roots meaning ghost/spirit or to see; it studies the interaction of matter with light across parts of the electromagnetic spectrum.
• Core idea: light interacts with a sample by absorption or reflection; different spectroscopic methods give different information about a sample.
• It can reveal information about molecular orientation, isomers, and other properties; effectively a powerful analysis tool for solutions, purity, quantification, and forensic applications.
• The term derives from the idea of “the shadow of a molecule”: we don’t see the molecule directly, but the light that has interacted with it.
• Everyday intuition: light is produced or scattered by a sample, and we measure that light to infer properties of the sample.

Basic physics background (energy levels and emission)

• Spectroscopy typically involves exciting molecules with light and monitoring transitions between energy levels.
• Ground state → excited state when energy (photon) is absorbed; relaxation returns to ground state, emitting light in the process.
• Emission mechanisms include fluorescence and phosphorescence; Raman scattering is mentioned but not covered in depth here.
• Emission can occur quickly (fluorescence) or more slowly (phosphorescence) depending on the states involved.
• For clarity: this framework is a qualitative backdrop; exam questions on detailed singlet/triplet states are not expected here.

Absorption and emission: key concepts

• Light interacts with a sample; depending on the molecular properties, the sample may absorb, scatter, reflect, refract, or transmit light.
• Absorption spectroscopy focuses on how the sample absorbs light, changing the transmitted light intensity.
• Emission spectroscopy (photoluminescence) focuses on light emitted by the sample after excitation:
  • Fluorescence: rapid emission that ceases when excitation stops.
  • Phosphorescence: slower emission following excitation.
• Spectra are typically plotted with wavelength (or energy/frequency) on the x-axis and intensity (absorbance or emission) on the y-axis.
• A spectrum can act as a fingerprint for components in a mixture; peak positions and intensities inform about identity and concentration.
• Beers’ Law (Beer-Lambert Law) is central to quantitative absorption:
  A = \varepsilon \, l \, c
  where A is absorbance, $\varepsilon$ is the molar extinction coefficient, $l$ is the path length, and $c$ is the molar concentration.
• Transmittance $T$ is related to absorbance by T = \frac{I}{I_0} = 10^{-A}, linking input light to transmitted light.

Absorption spectroscopy (UV-Vis) in practice

• Typical setup: light source → sample (e.g., a cuvette) → detector; sometimes a second excitation source is used.
• In practice, we measure how the sample modifies the incoming light (intensity) as a function of wavelength.
• When a sample absorbs light, fewer photons reach the detector at specific wavelengths, creating absorption peaks on the spectrum.
• Common outputs include identification of components in a solution and inference of concentrations via Beer's Law.
• Important practical notes:
  • Cuvette handling: avoid touching the optical sides to prevent extra absorption.
  • Turbidity (cloudiness) or poor mixing can affect results due to scattering and inhomogeneity.
  • Scattering, diffraction, reflection, and refraction are all potential deviations from ideal transmission.
  • Polarization and anisotropy can provide information about molecular orientation and isomeric form (e.g., racemic mixtures in pharmacology).
• Applications illustrated:
  • NAD/NADH kinetics: typical absorption peaks around ~260–280 nm; reduced NADH shows a peak around ~340–360 nm.
  • Hemoglobin spectroscopy: oxyhemoglobin vs deoxyhemoglobin have characteristic peaks; the Soret (surret) peak appears around ~400 nm (blue-green region) and shifts with iron binding state.
  • DNA quantification (A260/A280): absorbance at 260 nm for DNA concentration, and 280 nm to assess purity; ideal A260/A280 ratio for pure DNA is ~1.8.
• For DNA quantification, practical rule of thumb from the lecture:
  • Use A260 for concentration; use A260/A280 ratio to assess purity; target around 1.8 for DNA.
  • The technique applies to double-stranded or single-stranded DNA and RNA, with the same principle.

Photoluminescence: emission after absorption

• Photoluminescence is light emission after photon absorption, without requiring heat input.
• Subcategories include:
  • Fluorescence: rapid emission during or immediately after excitation; light emission usually ceases quickly once excitation stops.
  • Phosphorescence: delayed emission after excitation due to different relaxation pathways.
  • Chemiluminescence: light produced by a chemical reaction (no external light required, but often initiated by chemical energy).
• Real-world examples and concepts:
  • Fluorescence is commonly used in microscopy and diagnostics; phosphorescence is less common for routine labs but important historically and in certain materials.
  • Chemiluminescence examples: fireflies (luciferase-luciferin system), luminol in forensics.
• Key qualitative differences:
  • Fluorescence requires an external light source; emission stops when the source is removed.
  • Phosphorescence can persist after the light source is off due to slower decay.
• Practical measurement geometry for fluorescence:
  • Excitation beam is directed at the sample while detector looks at the emitted light at 90 degrees to minimize scattered excitation light.
  • A filter set helps isolate the emission signal; two spectra can be obtained: the excitation spectrum (emission measured as a function of excitation wavelength) and the emission spectrum (emission as a function of emission wavelength).
• Light sources commonly used:
  • Mercury lamp, xenon lamp, LEDs; lasers can also be used for excitation in some setups.
• Epifluorescence as a microscopy mode:
  • Uses illumination and detection through the same objective, enabling imaging of specimens.

Quantitative and qualitative fluorescence analysis

• Fluorescence can be used for both qualitative and quantitative analysis:
  • Qualitative: presence/absence, peak shapes, and relative intensities indicating purity or identity of analytes.
  • Quantitative: peak intensities or peak ratios can be correlated with concentration; standard solutions are used for calibration.
• Applications mentioned:
  • DNA identification, antibodies/antigens, drug analysis, pharmacokinetics, minerals and elements, amino acids.
  • Environmental testing: water and food contamination, soils.
• Advantages of fluorescence over UV-Vis:
  • Can be qualitative and quantitative; offers additional properties like polarization information for conformational analysis.
• Limitations and practical considerations:
  • Requires analytes that fluoresce or can be made to fluoresce; not suitable for turbid or non-fluorescent samples.
  • Instrumentation is more expensive than basic UV-Vis spectrophotometers.
• Important related concepts:
  • Polarization/isomer information (anisotropy) can be exploited to distinguish different molecular forms or racemic mixtures and to aid crystallography.
  • Stokes shift: the difference between the peaks of absorption and emission spectra; emission peak is shifted to longer wavelengths after absorption.
  • Correcting for source intensity and detector response is necessary to obtain the true absorption spectrum.
• Quenching and energy transfer:
  • Quenching: a process by which fluorescence is reduced or suppressed, often by a quencher in the sample.
  • Example: quinine fluorescence is quenched by chloride ions, illustrating that sample composition can influence fluorescence.
  • FRET (Förster Resonance Energy Transfer): energy transfer from a donor to an acceptor; depends strongly on the distance between donor and acceptor; used in signaling and imaging
  • FRAP (Fluorescence Recovery After Photobleaching): technique to study kinetics and diffusion in membranes; bleach a region and observe recovery over time to infer mobility.
• Fluorescence-based assays include bioluminescence and chemiluminescence variants:
  • Bioluminescence: luciferase-catalyzed light emission from luciferin, often requiring cofactors like calcium or magnesium; used in reporter gene assays to quantify biological activity.
  • Chemiluminescence: luminol-based reactions producing light (often used in forensics to detect trace blood); requires oxidation (e.g., hydrogen peroxide) and catalysts (e.g., iron).

Fluorescence microscopy and imaging techniques

• Epifluorescence microscopy: uses fluorescence to visualize samples; widely used but can suffer from out-of-focus light.
• Confocal microscopy: uses lasers and pinholes to acquire sharp, optical-sectioned images; enables 3D reconstruction.
  • Very powerful but expensive (cost ranges roughly from about $3,350 up to £500,000 for a confocal system).
• Fluorescent labeling strategies:
  • Antibody-based staining to target specific cellular components (multicolor panels demonstrated).
  • Visualization of multiple structures within cells (e.g., actin, membranes) using different fluorophores.
• Applications showcased:
  • Bacterial staining with multiple fluorescent probes to create complex images.
  • Near-infrared and multi-color fluorescence to distinguish different cellular components.

Near-infrared spectroscopy (NIR) and other advanced techniques

• NIR-absorbent spectroscopy used in fermentation and process monitoring to assess biologically important constituents in solution.
• NIR can also be used to analyze water content and quality; heavy water (D2O) shows subtle differences in spectra compared to ordinary water, enabling estimates of chemical composition.
• A note on water coloration in spectra: water appears blue due to the absorption of red part of the spectrum; high-level discussion of why water looks blue is mentioned.

Luminol chemiluminescence in forensics

• Luminol chemiluminescence detects traces of blood; reaction involves luminol, oxygen, and a catalyst (e.g., iron) in the presence of hydrogen peroxide.
• The reaction produces blue light as luminol is oxidized and returns to ground state.
• This process is widely used in forensics to visualize trace amounts of blood that are invisible to the naked eye.

DNA quantification and purity assessment using UV-Vis

• In practical DNA quantification, measure absorbance at 260 nm to estimate DNA concentration.
• Measure at 280 nm to assess protein contamination; common purity metric is the A260/A280 ratio.
• Target ratio for pure DNA is approximately 1.8; lower values indicate protein contamination or other impurities.
• This approach applies to double-stranded DNA, single-stranded DNA, and RNA, with interpretive caveats depending on the sample.

Practical takeaways and cautions

• Don’t rely on idealized conditions; real samples can be turbid, poorly mixed, or contain scattering/diffraction that affects measurements.
• Be mindful of instrumentation and sample preparation when interpreting spectra (filters, mirrors, excitation wavelengths, detector responses).
• Fluorescence techniques offer both qualitative and quantitative information but require appropriate analytes and instrumentation; they can be expensive and not always suitable for all samples.
• When planning experiments, think about what you want to measure (absorption vs emission), the sample’s properties (purity, concentration, fluorescence capability), and potential interferences.

Closing notes and references

• The speaker points to two textbooks (one specific to spectroscopy in Lenigen material) for more detailed information; these were mentioned as recommended resources for further reading.
• The session emphasized not needing deep physics mastery for exam readiness, but understanding excitation states, emission, and practical lab applications.

Quick glossary of terms mentioned

• Absorbance (A): measure of how much light is absorbed by a sample; linked to concentration via Beer's Law.
• Beer's Law / Beer-Lambert Law: A = \varepsilon \, l \, c
• Soret peak (surret peak): strong absorption band in the blue/violet region around ~400 nm, sensitive to heme state.
• Stokes shift: distance between the peaks of absorption and emission spectra.
• Fluorescence: fast emission of light following absorption, ceases when excitation stops.
• Phosphorescence: delayed emission after excitation.
• Quenching: reduction of fluorescence intensity due to interactions with other species.
• FRET (Förster Resonance Energy Transfer): energy transfer from donor to acceptor, distance-dependent.
• FRAP (Fluorescence Recovery After Photobleaching): measure of molecular mobility by bleaching a region and observing recovery.
• Epifluorescence: microscopy technique using fluorescence detected from above the specimen.
• Confocal microscopy: high-resolution 3D imaging using focused lasers and pinholes.
• NIR spectroscopy: near-infrared spectroscopy for analyzing molecular composition and processes.
• Luminol chemiluminescence: light emitted from a chemical reaction used in forensics to detect blood.
• DNA absorbance markers: A260 for concentration, A280 for purity, with A260/A280 ~ 1.8 indicating pure DNA.
• NADH/NAD+: metabolites with characteristic absorption peaks useful in enzyme kinetics studies.
• Anisotropy / polarization: properties of materials that vary with direction, useful for studying molecular orientation and crystallography.