Introduction
Enzyme-Linked Immunosorbent Assay (ELISA) is one of the most classic and widely used technology platforms in the immunoassay field. Since its development in the 1970s, ELISA has become the "gold standard" method for infectious disease screening, autoimmune testing, cytokine analysis, vaccine immunogenicity evaluation, and other applications due to its operational flexibility, controllable cost, and high throughput. The performance of high-quality ELISA reagents heavily depends on the proper selection and combination of antibody pairs, enzyme labels, chromogenic substrates, solid-phase carriers, blocking agents, stabilizers, preservatives, and other raw materials. this article systematically introduces the technical principles, main application areas, key raw material selection points, formulation examples, and frequently asked questions for ELISA, providing a complete raw material technical reference for ELISA reagent development.
I. Overview of ELISA Technology
1.1 Method Classification
ELISA is mainly divided into four types based on detection format and reaction principles:
| Method Type | Principle | Application Scenario | Characteristics |
|---|---|---|---|
| Direct Method | Antigen coating → Enzyme-labeled antibody directly detects | Antigen detection | Simple operation, but no signal amplification |
| Indirect Method | Antigen coating → Primary antibody binding → Enzyme-labeled secondary antibody detection | Antibody detection (e.g., HIV antibody) | Signal amplification, low cost |
| Sandwich Method | Capture antibody coating → Antigen binding → Enzyme-labeled detection antibody | Large molecule antigen detection (most common) | High sensitivity, strong specificity |
| Competitive Method | Antigen coating → Sample antigen competes with enzyme-labeled antigen for antibody binding | Small molecule antigen detection (e.g., hormones, drugs) | Suitable for projects where sandwich method cannot be used |
Most Common Formats: Sandwich method (antigen detection) and indirect method (antibody detection).
1.2 Principle of Double Antibody Sandwich Method (HRP/TMB System)
Steps:
1. Coating: Capture antibody adsorbed onto ELISA plate
2. Blocking: BSA blocks non-specific binding sites
3. Add sample: Target antigen binds to capture antibody
4. Add detection antibody: HRP-labeled detection antibody recognizes antigen
5. Color development: Add TMB substrate, HRP catalyzes color development (blue)
6. Stop: Add sulfuric acid, color turns yellow (450 nm reading)
II. Main Application Areas of ELISA
| Application Area | Specific Test Items | Format | Clinical Significance |
|---|---|---|---|
| Infectious Disease Screening | HIV antibody/antigen, HBsAg, HBcAb, HCV antibody, Syphilis antibody | Indirect/Sandwich | Blood screening, diagnosis |
| Autoimmune Testing | ANA (antinuclear antibody), ds-DNA, RF (rheumatoid factor), CCP | Indirect | Autoimmune disease diagnosis |
| Cytokine Testing | IL-6, TNF-α, IFN-γ, IL-1β | Sandwich | Inflammation research, immune monitoring |
| Vaccine Immunogenicity Evaluation | Neutralizing antibodies, IgG subtypes | Indirect/Sandwich | Vaccine potency assessment |
| Tumor Markers | AFP, CEA, PSA (partial) | Sandwich | Cancer screening |
| Food Safety | Allergens, veterinary drug residues, mycotoxins | Competitive | Food safety testing |
III. Key Raw Materials and Solutions for ELISA
3.1 Antigens and Antibodies — The "Soul" of Detection
The core of ELISA performance lies in the quality of the antibody pair. Whether for sandwich method or indirect method, antibody affinity, specificity, and pairing compatibility directly determine the sensitivity and specificity of the test.
| Raw Material Name | Function | Recommended Specification | Selection Points |
|---|---|---|---|
| Capture Antibody (Coating Antibody) | Solid-phase capture of target antigen | Affinity ≥10⁸ L/mol, high specificity | Must recognize different epitope from detection antibody |
| Detection Antibody (Enzyme-Labeled Antibody) | Recognizes target antigen, carries enzyme label | Affinity ≥10⁸ L/mol, HRP/AP labeled | Activity should not decrease after labeling |
| Standard/Control (Antigen) | Quantification standard | High purity, traceable to international standards | Matrix-matched, good stability |
Key Indicators for Antibody Pair Screening:
| Indicator | Explanation | Evaluation Method |
|---|---|---|
| Affinity | Strength of antibody-antigen binding | Surface plasmon resonance (SPR), ELISA titration |
| Specificity | No cross-reactivity with other proteins in the same family | Cross-reactivity experiment |
| Pairing Compatibility | Whether capture and detection antibodies interfere with each other | Checkerboard experiment |
| Lot-to-lot Consistency | Performance stability across different batches | Consecutive batch comparison test |
Antibody Labeling (HRP Labeling):
| Labeling Method | Principle | Advantages | Disadvantages |
|---|---|---|---|
| Glutaraldehyde Method | Glutaraldehyde links antibody and HRP | Simple operation, good reproducibility | May form multimers |
| Sodium Periodate Method | Oxidizes HRP sugar groups to link with antibody amino groups | High labeling efficiency | Complex operation |
| Commercial Labeling Kit | Pre-activated HRP for direct labeling | Simple, fast, standardized | Higher cost |
3.2 Enzymes and Substrates — The "Amplifiers" of Signal
The most commonly used enzyme-substrate system in ELISA is HRP + TMB, followed by AP + p-NPP.
| Raw Material Name | CAS No. | Function | Recommended Specification |
|---|---|---|---|
| HRP (Horseradish Peroxidase) | 9003-99-0 | Labels detection antibody, catalyzes substrate color development | RZ≥3.0, ≥250 U/mg |
| TMB (3,3',5,5'-Tetramethylbenzidine) | 54827-17-7 | Chromogenic substrate (blue → yellow) | ≥98%, low background |
| H₂O₂ (Hydrogen Peroxide) | 7722-84-1 | HRP substrate (oxidizing agent) | ≥30% |
| Sulfuric acid (Stop Solution) | 7664-93-9 | Stops reaction, stabilizes color | 1-2 M |
TMB Substrate Optimization Parameters:
| Parameter | Recommended Range | Impact |
|---|---|---|
| Buffer | Citrate-phosphate buffer (pH 4.0-5.0) | Maintains optimal reaction pH |
| TMB Concentration | 0.1-0.8 mM | Higher concentration gives higher sensitivity |
| H₂O₂ Concentration | 1.3-3.0 mM | Affects reaction rate |
| Stabilizers | 2-Hydroxy-β-cyclodextrin, DMSO | Enhances solubility and stability |
| Ions | Ca²⁺ (0.3-0.5 mM) | Enhances signal |
3.3 Solid-Phase Carrier — The "Stage" for Reaction
The ELISA plate (microplate) is the solid-phase carrier where antibody coating and reactions occur. Its quality directly affects coating efficiency and result reproducibility.
| Raw Material Name | Function | Recommended Specification | Selection Points |
|---|---|---|---|
| High-Binding ELISA Plate | Passive adsorption of coating antibodies/antigens | Binding capacity ≥300 ng/cm² | Suitable for most ELISAs |
| Medium-Binding ELISA Plate | Passive adsorption, reduces background | Binding capacity 150-200 ng/cm² | Use when low background is required |
| Streptavidin Pre-coated Plate | Captures biotinylated antibodies | Loading capacity ≥5 pmol/well | Signal amplification system |
ELISA Plate Selection Guide:
| Application Scenario | Recommended Plate Type | Rationale |
|---|---|---|
| Routine ELISA (high sensitivity required) | High-binding plate | Maximum antibody coating |
| High-throughput screening | Standard 96-well plate | Compatible with automatic washers/readers |
| Low background requirement | Medium-binding plate | Reduces non-specific adsorption |
| Biotin-streptavidin system | Streptavidin pre-coated plate | Oriented coating, better consistency |
3.4 Blocking Agents — The "Barrier" for Reducing Background
Blocking agents occupy non-specific binding sites on the ELISA plate, preventing detection antibodies or sample proteins from directly adsorbing to the plate bottom, thereby reducing background signal.
| Raw Material Name | CAS No. | Function | Recommended Concentration | Advantages | Disadvantages |
|---|---|---|---|---|---|
| BSA (Bovine Serum Albumin) | 9048-46-8 | Blocks non-specific binding sites | 1-3% | Common, low cost | May contain IgG, causing cross-reaction |
| Non-fat Dry Milk | — | Blocks non-specific binding sites | 2-5% | Strong blocking ability | Contains biotin, interferes with certain systems |
| Casein | 9000-71-9 | High-efficiency blocking | 0.5-2% | Strong blocking effect | Poor solubility |
| Gelatin | 9000-70-8 | Mild blocking | 0.5-2% | Does not interfere with immune reactions | Incomplete blocking |
Blocking Agent Selection Principles:
-
Routine ELISA: BSA (1-3% in PBS) is the first choice
-
High-sensitivity detection: Use fatty acid-free, low endotoxin BSA
-
Biotin-containing systems: Avoid non-fat dry milk (contains biotin)
-
Plate stabilizer treatment: After blocking, add plate stabilizer for dry storage
3.5 Buffers and Wash Solutions — The "Environment" for Reaction
Buffers provide a stable pH and ionic environment for each ELISA step, while wash solutions remove unbound substances to reduce background.
| Raw Material Name | Function | Recommended Formulation | Key Parameters |
|---|---|---|---|
| Coating Buffer (CBS) | Promotes antibody/antigen adsorption | Carbonate buffer, 50 mM, pH 9.6 | pH 9.4-9.8 |
| Wash Buffer (PBST) | Washes away unbound substances | PBS + 0.05-0.1% Tween-20 | Tween-20 concentration affects wash intensity |
| Blocking Buffer | Blocks non-specific binding sites | PBS + 1-3% BSA | Can be adjusted as needed |
| Sample Diluent | Dilutes samples, reduces matrix effects | PBS + 0.5-1% BSA + 0.05% Tween-20 + preservative | Matrix-matched |
| Antibody Diluent | Dilutes detection antibody | PBS + 1% BSA + 0.05% Tween-20 | Can add stabilizers |
| Stop Solution | Stops enzyme reaction | 1-2 M H₂SO₄ | Concentration affects color stability |
Tween-20 Concentration Selection:
| Concentration | Wash Intensity | Applicable Scenarios |
|---|---|---|
| 0.05% | Mild | Routine ELISA, low background |
| 0.1-0.2% | Moderate | Use when background is high |
| 0.5% | Strong | Special applications (stringent washing) |
3.6 Stabilizers and Preservatives — The "Protectors" of Reagent Life
| Raw Material Name | CAS No. | Recommended Concentration | Main Application | Precautions |
|---|---|---|---|---|
| Trehalose | 6138-23-4 | 2-5% | Lyoprotection, thermal stability | Extends shelf life |
| Glycerol | 56-81-5 | 5-20% | Cryoprotection | Used for enzyme stock solutions |
| Sucrose | 57-50-1 | 5-10% | Protectant | Commonly used |
| ProClin 300 | — | 0.02-0.05% | Preservative (aqueous reagents) | Compatible with HRP |
| Sodium azide | 26628-22-8 | — | ⚠️ Prohibited for HRP reagents | Inhibits HRP activity |
Plate Stabilizer:
-
Used for dry storage of coated plates
-
Treated plates can be stored at 2-8°C for 6-12 months
-
Effectively blocks excess sites, protects coating antibody activity
IV. ELISA Formulation Examples
4.1 Coating Buffer Formulation (Carbonate Buffer, CBS)
| Component | Concentration | CAS No. | Function |
|---|---|---|---|
| Sodium carbonate (Na₂CO₃) | 15 mM | 497-19-8 | Adjusts pH |
| Sodium bicarbonate (NaHCO₃) | 35 mM | 144-55-8 | Adjusts pH |
| Deionized water | — | — | Solvent |
| Final pH | 9.6 | — | — |
4.2 Wash Buffer Formulation (PBST, 10× Concentrate)
| Component | Concentration (10×) | CAS No. | Function |
|---|---|---|---|
| Sodium chloride (NaCl) | 1.37 M | 7647-14-5 | Ionic strength |
| Potassium chloride (KCl) | 27 mM | 7447-40-7 | Ionic strength |
| Disodium hydrogen phosphate (Na₂HPO₄) | 100 mM | 7558-79-4 | Buffer |
| Potassium dihydrogen phosphate (KH₂PO₄) | 18 mM | 7778-77-0 | Buffer |
| Tween-20 | 0.5-1% | 9005-64-5 | Detergent |
| Final pH | 7.4 | — | — |
| Working dilution | 1:10 dilution | — | — |
4.3 Blocking Buffer Formulation (1× PBS + 3% BSA)
| Component | Concentration | CAS No. | Function |
|---|---|---|---|
| BSA | 3% | 9048-46-8 | Blocks non-specific binding sites |
| PBS (1×) | Balance | — | Buffer environment |
| ProClin 300 | 0.02% | — | Preservative (optional) |
4.4 Sample/Antibody Diluent Formulation
| Component | Concentration | Function |
|---|---|---|
| PBS (1×) | Base buffer | Buffer environment |
| BSA | 0.5-1% | Stabilizes proteins, reduces adsorption |
| Tween-20 | 0.05% | Reduces non-specific adsorption |
| ProClin 300 | 0.02% | Preservative |
4.5 TMB Substrate Solution Formulation (Optimized)
Solution A (Citrate-Phosphate Buffer + H₂O₂):
| Component | Concentration | Function |
|---|---|---|
| Citric acid | 50 mM | Buffer |
| Disodium hydrogen phosphate | 100 mM | Buffer |
| H₂O₂ | 1.3 mM | HRP substrate |
| Final pH | 4.5 | — |
Solution B (TMB Chromogen Solution):
| Component | Concentration | Function |
|---|---|---|
| TMB | 0.8 mM | Chromogenic substrate |
| DMSO | 5% | Solubilizer |
| 2-Hydroxy-β-cyclodextrin | 0.4 mM | Stabilizer |
Use: Mix Solution A and Solution B in equal ratio (prepare immediately before use)
4.6 Stop Solution Formulation
| Component | Concentration | Function |
|---|---|---|
| Concentrated sulfuric acid (H₂SO₄) | 1-2 M | Stops HRP reaction, color changes from blue to yellow |
Preparation Note: Slowly add concentrated sulfuric acid to water (acid into water) to avoid splashing.
V. ELISA Procedure Example (Sandwich Method)
| Step | Operation | Conditions | Key Parameters |
|---|---|---|---|
| 1. Coating | Capture antibody diluted in CBS (1-10 μg/mL), 100 μL/well | 4°C overnight or 37°C 2 hours | Coating amount affects sensitivity |
| 2. Wash | Wash 3 times with PBST, 300 μL/well | Let sit 1-2 minutes each time | Insufficient washing → high background |
| 3. Blocking | Add blocking buffer (3% BSA), 200 μL/well | 37°C 1-2 hours | Insufficient blocking → high background |
| 4. Wash | Wash 3 times with PBST | Same as above | — |
| 5. Sample addition | Add standards/samples to blocked wells | 37°C 1-2 hours | Antigen-antibody binding |
| 6. Wash | Wash 3-5 times with PBST | — | Critical step |
| 7. Add detection antibody | HRP-labeled detection antibody (0.1-1 μg/mL), 100 μL/well | 37°C 1 hour | Antibody concentration needs optimization |
| 8. Wash | Wash 3-5 times with PBST | — | Critical step |
| 9. Color development | Add TMB substrate solution, 100 μL/well | Room temperature/37°C, 10-30 minutes | Protect from light, observe blue color change |
| 10. Stop | Add 1-2 M H₂SO₄, 50 μL/well | — | Color changes from blue to yellow |
| 11. Read | Microplate reader reads OD₄₅₀ | — | Can reference 620-650 nm |
VI. Frequently Asked Questions (FAQ)
Q1: Why is the background too high in my ELISA experiment?
A: High background is usually related to the following factors:
| Cause | Solution |
|---|---|
| Insufficient blocking | Increase blocking agent concentration (BSA 3-5%) or extend blocking time (room temperature 2 hours or 4°C overnight) |
| Insufficient washing | Increase wash cycles (5 times) or increase Tween-20 concentration (0.1-0.5%) |
| Non-specific antibody binding | Replace antibody pair, or add 1% normal serum to antibody diluent |
| Plate quality issues | Use high-binding plates, ensure plates are not damaged |
| Reagent contamination | Check preservative effectiveness; use ProClin 300 |
Q2: How can I improve low sensitivity in ELISA?
A: Low sensitivity is usually related to antibodies, blocking, substrates, or incubation conditions.
| Cause | Solution |
|---|---|
| Insufficient antibody affinity | Select high-affinity antibodies with affinity ≥10⁸ L/mol |
| Coating antibody concentration too low | Optimize coating concentration; perform checkerboard experiment |
| Blocking agent interference | Change blocking agent (e.g., from milk to BSA) |
| Detection antibody concentration too low | Increase detection antibody concentration |
| Low TMB substrate sensitivity | Optimize TMB formulation (pH 4.5, add stabilizers) |
| Insufficient incubation time | Extend incubation time (e.g., from 1 hour to 2 hours) |
| Incorrect incubation temperature | Use 37°C incubation instead of room temperature |
Q3: How can I reduce high CV (Coefficient of Variation) in ELISA?
A: High CV is usually related to operational consistency or plate quality.
| Cause | Solution |
|---|---|
| Inaccurate pipetting | Use calibrated pipettes; maintain consistent pipetting angle |
| Inconsistent washing | Use an automatic plate washer; or maintain consistent angle when manual aspirating |
| Edge effect | Seal with plate sealing film; avoid stacking during incubation |
| Reagents not mixed | Mix gently before use; avoid bubbles |
| Sample matrix interference | Use matched sample diluent |
Q4: Weak or no TMB color development in ELISA, what should I do?
A: Color development issues are usually related to enzyme activity, substrate, or stop solution.
| Cause | Solution |
|---|---|
| HRP label inactive | Check HRP label storage conditions (2-8°C, protect from light, sodium azide prohibited) |
| TMB substrate expired | Confirm TMB has not changed color (should be colorless or very light blue); expired product cannot be used |
| H₂O₂ expired | Prepare fresh or aliquot for storage |
| Incorrect stop solution concentration | Use 1-2 M H₂SO₄ |
| Incorrect reaction pH | Ensure TMB substrate pH is 4.0-5.0 |
Q5: Can I use sodium azide as a preservative for ELISA reagents?
A: No. HRP-containing ELISA reagents must NOT use sodium azide.
-
Sodium azide (CAS: 26628-22-8) strongly inhibits HRP activity
-
HRP is the key enzyme in ELISA (HRP-TMB system)
Recommended alternative: ProClin 300 (0.02-0.05%) — broad-spectrum antimicrobial, compatible with HRP.
Q6: How can I extend the shelf life of ELISA coated plates?
A: Use plate stabilizer for post-treatment of coated plates.
| Step | Operation |
|---|---|
| 1. Coating | After blocking, aspirate blocking buffer |
| 2. Add plate stabilizer | Add stabilizer (200 μL/well) |
| 3. Incubate | Incubate at room temperature for 30 minutes |
| 4. Remove stabilizer | Tap dry; do not wash |
| 5. Dry | Vacuum dry or dry at 37°C for 1-2 hours |
| 6. Package | Place in aluminum foil bag with desiccant, seal |
| 7. Store | Store at 2-8°C for 6-12 months |
VII. Summary
ELISA, as the most classic and widely used technology platform in immunoassay, has reagent performance that heavily depends on the proper selection and combination of core raw materials.
艳 李
