When the sample is injected into the microfluidic chip, it is separated or processed by the system, and the corresponding assay is needed to determine the relevant components and their contents. Microfluidic chips are highly regarded for their small size, low sample consumption, high degree of integration and rapid analytical capabilities.
Therefore, the requirements for detection technologies include easy integration, miniaturization, fast response and high sensitivity. Currently, more than ten detection methods have been designed specifically for microfluidic chips, of which optical and electrochemical detection are particularly common and effective.
1. Optical inspection
1) Fluorescence Detector
Laser-induced fluorescence detector and LED fluorescence detector are two common detection methods. Among them, laser-induced fluorescence detector is the earliest and most widely used method in microfluidic chip systems, which has high sensitivity and is suitable for the detection of single molecules with small volume and low reagent amount.
Many substances to be tested, such as DNA, amino acids and proteins, can be recognized by fluorescent labeling. Studies have been conducted to develop a fluorescent immunoassay platform based on ZnO nanostructured arrays for early diagnosis and prognosis of different cancers by using static droplet arrays.
2) Absorption Spectrum Detector
Absorption spectroscopy detection is a common method in optical detection, but the sensitivity may be limited due to the small detection area and short optical range of microfluidic chips. In addition, this technique requires certain chip materials, which limits its application in microfluidic chips to some extent.
Combining UV-visible spectrophotometry and microscopic Raman spectroscopy, the researchers have succeeded in realizing the analysis of separate active substances in microfluidic sampling devices, opening up new possibilities for the analysis of more complex mixtures of aqueous solutions.
3) Chemiluminescence detector
The heart of the assay lies in the fact that the concentration of the substance to be measured and the intensity of the chemiluminescence produced by the system show a direct linear relationship under specific conditions. By measuring the intensity of the chemiluminescence, the amount of the substance to be measured can be precisely calculated.
It includes two methods, ordinary chemiluminescence detection and electrochemical detection. This method has the advantages of high sensitivity, simple equipment and easy integration, and is widely used in the detection of microfluidic chip technology.
4) Laser thermal lens detector
By keeping the intensity of the excitation light constant and monitoring the changes in the intensity of the detection light, the concentration of the sample solution can be measured indirectly. A study of an optofluidic laser-based dopamine sensor showed that the biosensor had a detection limit of 0.3 μM and exhibited good selectivity in dopamine detection.
5) Emission Spectrum Detector
Plasma emission spectroscopy detection and atomic emission spectroscopy detection are two common detection methods.
Plasma Atomic Emission Spectrometry is a highly sensitive means of inorganic analysis based on the basic principle that when a sample is excited by a plasma generated by a high-frequency, high-voltage current, the elements and compounds in the sample emit light of a specific wavelength and intensity. By measuring the wavelength and intensity of this light, the amount of each element and compound in the sample can be calculated.
Atomic emission spectroscopy, on the other hand, is used for qualitative and quantitative analysis of elements by measuring the distinctive spectral lines emitted by the atoms of the element under test as they fall back from the excited state to the stable state. However, this method is mainly used for elemental analysis and does not provide information on the specific morphology and structure of the element.
The detection of non-metallic substances also presents challenges, as their detection sensitivity is usually low. In addition, the instruments and equipment required for such methods are complex and costly.
6) Refractive index detector
An oscillometric refractive detector is a widely used detection device that responds to different substances based on changes in their refractive index. The detector works effectively as long as there is a difference in the refractive index between the target component and the eluent. In the field of life sciences, faced with a wide range of saccharides and aliphatic compounds, the oscillometric refractive detector is frequently used because of its broad applicability.
The experimentalists also developed a 3D-printed optofluidic fiber optic sensor for real-time refractive index measurement. The sensor enables remote real-time refractive index measurements without the need for sample pre-processing, providing theoretical support for more accurate real-time monitoring of the refractive index of dynamic fluids.
2. Electrochemical detection
Electrochemical detection, as a commonly used analytical method, efficiently converts the chemical information of target components in solution into electrical signals by using electrodes as sensing devices to achieve accurate detection of these components.
Electrochemical detectors have the advantages of high sensitivity, compact size, simple operation and low cost, which are suitable for miniaturized processing and integrated design. It is these unique advantages that make electrochemical detection an ideal detection method in microfluidic systems.
1) Amperometry
The amperometric method induces a redox reaction in the target substance by applying a constant potential to the working electrode, and then quantitatively analyzes the substance content by measuring the resulting oxidation or reduction current.
This technique has high sensitivity similar to laser-induced fluorescence and exhibits good selectivity. However, small current fluctuations in the environment may affect the accuracy of amperometric measurements, so it needs to be operated in a very stable environment.
2) Conductivity
The conductivity method analyzes samples by monitoring changes in the conductivity of a solution and does not depend on the electrochemical reaction of the substance under test on the surface of the electrode. Because of its broad applicability, this method is particularly suitable for detecting tiny ions that are difficult to analyze by conventional methods, such as inorganic salts and amino acids.
3) Potential method
Potentiometric detection techniques analyze samples by measuring the potential difference between an indicator electrode and a reference electrode. Miniature ion-selective electrodes are typically used, and the application of this method in chip electrophoresis is relatively limited due to the lack of response of the substrate solution to the electrode.
3. Mass spectrometry
Mass spectrometry detection techniques are based on the principles of electromagnetism, which enables qualitative and quantitative analysis by separating charged sample ions and differentiating them according to their mass-to-charge ratio. Electrospray ionization and matrix-assisted laser-resolved ionization are two commonly used soft ionization techniques that are widely used in the study of biomolecules such as peptides and proteins.
For complex functional chip systems, a large number of pipes are usually required to connect modules such as pumps, valves and mass spectrometers, which increases the instability and error of the system and limits the large-scale application of microfluidic mass spectrometry detection. However, with the improvement of mass spectrometry detection sensitivity and the optimization of chip design, mass spectrometry detection still has a broad application prospect and good technical feasibility in the field of microfluidic chip analysis.
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