Covalent Conjugation of Antibodies to Gold Nanoparticles Tutorial

Gold nanoparticles exhibit fundamentally different properties compared to bulk gold due to quantum confinement effects. In the 10-200 nm range, collective electron oscillations (plasmon resonance) cause gold to display colors from red to blue depending on particle size and shape. Spherical particles appear red, while rod-shaped particles show additional absorption peaks due to longitudinal and transverse plasmon modes. Commercial products labeled identically may have different morphologies—TEM reveals variations in particle shape (spheres, rods, triangles, fused particles) even among 40 nm samples. Absorbance spectroscopy provides quantitative information but requires careful interpretation—mixing different-sized gold nanoparticles produces a single averaged peak rather than additive profiles. Aggregation detection uses absorbance ratios (e.g., 650/530 nm) in the shape-sensitive region. Surface chemistry is critical: molecular self-assembly with alkane thiols creates organic layers but provides limited stability. Overcoat technology wraps protective layers around entire particles, providing exceptional colloidal stability and resistance to extreme conditions (70°C, 2.5M NaOH).

Colloidal gold nanoparticles are synthesized by reducing gold compounds with citrate ions in a controlled heating process. The reaction produces particles 10-100 nanometers in diameter—100 times smaller than smoke particles—that form stable suspensions. Particle size determines optical properties through surface plasmon resonance, causing solutions to display colors from blue through purple to brilliant red. Adding salt causes aggregation into larger particles that reflect light more intensely. These nanoparticles have applications in medicine, diagnostics, and advanced materials science.

This section addresses colloidal stability and protection strategies. Lyophilic colloids are highly stable due to solvation/hydration and require specific conditions for coagulation. Lyophobic colloids are inherently unstable and protected by adding lyophilic colloids that form protective layers. The gold number quantifies protective power—the minimum milligrams of protective colloid needed to prevent coagulation of gold sol by 1ml 10% NaCl. Lower gold numbers indicate superior protection capability.

This segment covers surface chemistry and colloidal systems. The instructor explains: (1) The nature of surface phenomena and adsorption, (2) How to characterize colloidal systems, (3) The concept of colloidal stability and flocculation, (4) The applications of surface chemistry in industrial processes. The instructor emphasizes that understanding surface chemistry is essential for understanding phenomena like catalysis, corrosion, and the behavior of colloidal systems.

Brownian movement is the random zigzag motion of colloidal particles caused by bombardment with dispersion medium molecules. Active centers on catalyst surfaces (steps, cracks, corners) are coordinately unsaturated sites where adsorption occurs. The electrical double layer forms when colloidal particles preferentially absorb ions, attracting oppositely charged ions from the medium. Gold number measures protective power of hydrophilic colloids: smaller gold number indicates greater protective power.

Bioconjugation links small drug molecules to carrier proteins via amide bonds using EDC/NHS chemistry. EDC activates carboxylic acids to form unstable O-acylisourea intermediates, which react with NHS to create amine-reactive NHS esters that bind to protein amines. The complete workflow involves: (1) Calculating concentrations to achieve targets like 20 mg/mL; (2) Preparing EDC and NHS in 20 equivalent amounts; (3) Preparing buffer solutions (1X and 5X PBS); (4) Preparing carboxylic acid solution in DMSO with pH adjustment to slightly acidic (4.5-5.0) using 1M HCl; (5) Combining all components and performing activation on a rotator for 2.5 hours. This systematic approach enables reproducible bioconjugation for therapeutic applications.

Three fundamental reactions enable antibody conjugation: (1) NHS ester-amine reaction (pH 8) for fluorescent dye labeling—80% of bioconjugations use this direct reaction with lysine residues, but NHS esters are unstable and moisture-sensitive; (2) Carboxylic acid-amine coupling via EDC (pH 6) for carboxylated particle labeling—EDC activates carboxylic acids to react with lysine amines, though cross-linking can occur; (3) Maleimide-thiolation for introducing functional groups—requires prior antibody modification but preserves antibody charge and structure. These three reactions provide sufficient knowledge to conjugate any antibody to any label without needing to master all conjugation chemistries.

The EDC (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide) coupling reaction provides an alternative approach to surface functionalization by coupling carboxylic acid groups with primary amines to form amide bonds. Unlike NHS reactions, EDC acts as a catalyst that activates carboxylic acids, enabling rapid reaction completion within one hour. This method is particularly valuable for attaching amino acids (which all contain carboxylic acid groups) and diverse functional groups including thiols, benzene rings, and various organic moieties. A distinctive feature of this video is the demonstration of gold surface functionalization without requiring pre-existing surface functional groups—compounds containing thiol groups can bind directly to gold through strong gold-sulfur bonds. This direct binding capability makes gold an ideal substrate for advanced applications in nanotechnology, biosensing, and surface engineering.

Covalent bioconjugation creates stable attachments between biomolecules and surfaces. EDC/NHS chemistry activates carboxyl groups for amine coupling. Photoreactive methacrylation enables UV-triggered crosslinking between methacrylated surfaces and biomolecules. Maleimide-biotin systems exploit the strongest known non-covalent interaction (avidin-biotin) for rapid conjugation. Each method requires identifying appropriate reactive groups on both partners. Targeting groups far from protein active sites preserves bioactivity. These techniques enable creation of stable, long-lasting bioactive surfaces for biomedical applications.

Bioconjugation exploits diverse functional groups across biomolecules: proteins offer amines (alpha, lysine), carboxylates (aspartic, glutamic acid), thiols (cysteine), guanidinium (arginine), phenolics (tyrosine), and imidazoles (histidine); carbohydrates provide aldehydes and oxidizable hydroxyls; nucleic acids offer phosphate groups and base positions. Three major reaction classes dominate: amine reactions using NHS/TFP esters form stable amide bonds; thiol reactions via maleimides provide rapid cysteine targeting; carbonyl reactions (aldehydes, ketones) enable orthogonal conjugation through oximes/hydrazones. NHS esters represent the most prevalent reactive group, forming stable amide bonds through nucleophilic attack by amines at pH 7-9. However, NHS esters face significant challenges: hydrolysis accelerates at alkaline pH, with half-lives dropping from hundreds of minutes at pH 3-4 to under 10 minutes at pH 8; they react with histidine imidazole groups and serine/threonine/tyrosine hydroxyls under certain conditions. TFP esters offer improved hydrolytic stability and yields compared to NHS esters, particularly at alkaline pH. EDC activates carboxylates directly in aqueous solution through a multi-step mechanism involving protonation, hydrolysis to iso-urea, reaction with carboxylic acid, and formation of a reactive carbocation. EDC is most effective at pH 3.5-4.5, though it remains stable at pH 6. One-step EDC reactions yield poor amide bond formation because EDC hydrolyzes rapidly at optimal conjugation pH. Two-step protocols where excess EDC is removed before adding the amine-containing molecule provide significantly better yields. Using sulfo-NHS to convert EDC-activated esters to sulfo-NHS esters dramatically improves yields by preventing hydrolysis and rearrangement side reactions.

Lateral flow assays (like rapid diagnostic tests) work by detecting specific antigens or antibodies in a sample. The sample flows along a test strip containing immobilized antibodies. If the target antigen is present, it binds to the test line antibodies, producing a visible line. A control line always appears to confirm the test is working. This principle allows for rapid, point-of-care diagnostic testing.

Lateral flow assays use capillary action to move samples across porous strips containing antibody spots. The sample first encounters capture antibodies that bind target antigens. The antigen-antibody complex then encounters labeled antibodies (with enzymes, fluorescent tags, or colored particles) that bind to the same antigen, creating a sandwich complex. Unbound labeled antibodies continue to the control line. Two visible lines (test and control) indicate positive result; only control line indicates negative.

Lateral flow assays are rapid, low-cost immunoassays that detect specific targets (like infectious diseases, allergens, or hormones) through antibody-antigen interactions; the test works by applying a sample to a device where labeled detection antibodies bind to antigens if present, then migrating by capillary action to form a sandwich complex with capture antibodies at the test line while unbound antibodies bind to control antibodies, producing visible colored lines indicating positive or negative results.

Lateral flow assays consist of four functional zones: sample pad for loading, conjugate pad containing mobile antibodies, test line with immobilized capture antibodies, and absorbent pad for liquid management. The assay operates on sandwich immunoassay principles where target antigens bind to primary antibodies in the conjugate pad, then continue moving until captured by immobilized secondary antibodies at the test line. The control line ensures test validity by detecting unbound primary antibodies. Positive results show color in both lines; negative results show only the control line. Any deviation indicates an invalid test.

Lateral flow immunoassays are unidirectional diagnostic tests that detect analytes through antibody-antigen interactions, consisting of five key components: a sample application pad, conjugate release pad containing detection particles (gold nanoparticles or latex beads), reaction membrane with immobilized capture antibodies, wicking pad, and optionally a plastic cassette. The assay operates in two formats: direct assay for large analytes with multiple antigenic sites (producing two lines indicating positive results) and competitive assay for small analytes with single binding sites (producing only a control line for positive results). Successful assay development requires selecting antibodies with high binding affinity (low KD value) and fast on-rates, as well as ensuring uniform detection particle size and stable conjugation methods. Common troubleshooting issues include uneven lines, false positives/negatives, and inconsistent liquid fronts, which can be resolved through component optimization and quality control measures.