ELISA Reader Principles & Selection Guide: Absorption, Fluorescence & Luminescence

Introduction to ELISA Reader Principles

In the vast world of life science research, ELISA readers (microplate readers) are not only indispensable partners in completing experiments, but also core engines driving research and quality control. From basic absorption detection to complex fluorescence luminescence applications, correctly selecting an ELISA reader is not only crucial for data reliability, but also a strategic decision for improving research efficiency and ensuring production stability.

So, faced with a dazzling array of instrument models and complex application scenarios, how do we make an informed choice? This article will guide you from principles to selection, taking you through the process!

01 Absorption

Principle: A beam of monochromatic light passes through a sample, part of which is absorbed, and part is transmitted. Absorbance is the amount of light absorbed by the sample.

Applications: ELISA, nucleic acid/protein quantification, etc.

Recommended plate type: Transparent plate or UV plate.

02 Luminescence

Principle: Light is generated through a chemical reaction (such as firefly bioluminescence), without the need for external light source excitation.

Applications: Reporter gene detection, ATP analysis, immunoluminescence, etc.

Recommended Plate Type: White plate or transparent plate with white border—helps reflect light signals and improve sensitivity.

03 Fluorescence

Principle: Specific wavelength light excites fluorescent groups, causing them to emit longer wavelength light.

Applications: Cell activity, gene expression, fluorescence immunoassay.

Recommended Plate Type: Black plate or transparent plate with black border—reduces background interference and inter-well crosstalk.

Advanced Fluorescence Detection Techniques

01 Fluorescence Polarization (FP)

Principle: Detects the strength of the interaction by analyzing the change in the rotational velocity of the fluorescently labeled molecule before and after binding to a macromolecule. After binding, the molecule becomes larger, and the polarized light signal is enhanced.

Applications: Receptor-ligand interactions, kinase activity, competitive immunoassay.

02 Time-Resolved Fluorescence (TRF)

Principle: Utilizes the long fluorescence lifetime of lanthanides to delay detection and avoid short-lived background fluorescence.

Applications: GPCR research, kinase detection, biomarker analysis, drug screening.

03 Alpha:

Principle: Utilizes the diffusion of singlet oxygen to trigger luminescence between closely spaced microbeads, detecting biomolecular interactions.

Applications: Protein-protein interactions (PPI), GPCR signaling, epigenetic screening.

04 Fluorescence Resonance Energy Transfer (FRET)

Principle: Due to the overlap between the donor's emission spectrum and the acceptor's absorption spectrum, when the donor and acceptor are very close, the donor is excited and transfers energy to the acceptor, causing the acceptor to emit fluorescence at its characteristic wavelength.

Applications: Studying protein-protein and protein-nucleic acid interactions, protein folding, etc.

05 BRET (Bioluminescence Resonance Energy Transfer)

Principle: Similar to FRET, but the donor is a bioluminescent protein, requiring no excitation light and exhibiting extremely low background.

Applications: Interaction studies, acceptor activation analysis, etc.

ELISA Reader Selection

When selecting an ELISA reader, please be sure to clarify:

Core Application Type (Is it for high-end interactive research? Or routine quantitative analysis?)

Future Expandability (Will it be expanded to include luminescence, fluorescence, etc. detection?)

Detection Unit

The energy difference before and after light passes through the analyte is the energy absorbed by the analyte. At a specific wavelength, the concentration of the same analyte is quantitatively related to the absorbed energy.

The detection unit is expressed as OD value. OD is an abbreviation for optical density, representing the light density absorbed by the analyte. OD = Log(1/trans), where trans is the transmittance of the analyte. According to the Bouger-Amber-T-Beer rule, the OD value is related to the light intensity as follows:

E = OD = log10/1. where E represents the absorbed light density, 10 is the light intensity before the analyte, and 1 is the light intensity after the analyte.

The OD value is calculated using the following formula:

E=OD=C×D×E

C is the concentration of the analyte

D is the thickness of the analyte

E is the molar factor

Each substance has a specific wavelength at which it absorbs the most light energy. Choosing other wavelengths will result in inaccurate detection results. Therefore, when measuring analytes, we select a specific wavelength for detection, called the measurement wavelength.

However, each substance also exhibits a certain degree of non-specific absorption of light energy. To eliminate this non-specific absorption, we select a reference wavelength to eliminate this inaccuracy. At the reference wavelength, the absorption of light by the analyte is minimal. The difference between the absorbance values of the detection wavelength and the reference wavelength can eliminate non-specific absorption.

ELISA reader detection value calculation

The detector in the instrument receives the light energy transmitted through the analyte and converts it into a binary digital signal, with a maximum value of 4095. The instrument defines the transmittance value as 0% in the absence of a light source and 99.9% in the absence of analytes. In actual testing, the transmittance of the analyte is between 0% and 99.9%. The transmittance is calculated as follows:

T = (Meas - Min) / (Max - Min)

Where T is the transmittance, Meas is the binary value of the detected sample, Min is the binary value detected at 0%, and Max is the binary value detected at 99.9%. For example:

Max = 3600. Mn = 20. Meas = 30

T = (30 - 20) / (3600 - 20) = 0.0028

OD = log(1/T) = log(1/0.0028) = 2.552

Centering of the Microplate Reader

The instrument automatically centers the microplate wells. Centering eliminates inaccuracies caused by uneven thickness at the bottom of the wells. When performing tests on each ELISA reader, the instrument actually measures 35 points, and the average of the five middle points is taken as the OD value of that well.

Reference Channel for Light Source

The reference channel is used to calibrate against the effects of voltage instability or bulb wear.

Applications and Other Notes of the ELISA Reader

Used for ELISA reagent assays, widely used in various laboratories, including clinical laboratories.

Quality Control

Quality control is a crucial factor in reagent testing. Please perform quality control according to the reagent instructions.

Blank Correction

Some kit instructions set the blank wells to air; most others use reagent settings. Please follow the kit instructions.

Interpretation of Test Results

Since many factors can affect test results, such as different ELISA plates and reagent volumes, OD values will vary. Therefore, only test results from reagents reacted on the same ELISA plate can be compared and analyzed. For clinical interpretation of results, please refer to the kit instructions.