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It is no surprise that technological advancements on the hardware and software used in smartphones hasd been improving at a rapid pace. These advancements have not only allowed for a new generation of smartphones to be incredibly powerful, but they have also improved the user-friendly nature of these devices.
In particular, the innovations made on the complementary metal oxide-based semiconductors (CMOS) based cameras have supported the development of various optical biosensors. Recently, various researchers have successfully combined imaging and spectrometry-based techniques to create smartphone biosensors that could also be applied for biosensing and diagnostic purposes.
The Rise of Smartphone Biosensors
The affordability, portability and ease of updating applications, which are more commonly referred to as ‘apps,’ on smartphone devices, sparked interest in researchers to investigate whether the same technologies used in smartphones could be viable options for the complete or partial replacement of traditional laboratory devices, particularly those that are expensive and space-consuming. As a result of their improved performance, smartphones have been widely used as biosensing devices that can also function as data processors, detectors and signal inducers. Several other functionalities can be incorporated into smartphones by using toggles or attachments, such as smartphone card reader attachments. Researchers have therefore postulated that smartphones powered by these advanced biosensors could potentially play a role in improving point-of-care testing (POCT) technologies for future diagnostic tools.
Laboratory Techniques Based on Spectrometry
There are currently various types of laboratory techniques that utilize optical biosensors to detect minute quantities of analytes, some of which include lateral flow immunoassay (LFIA), enzyme-linked immunosorbent assay (ELISA), chemiluminescence and electrochemiluminescence. These techniques, which are primarily based on calorimetric and spectrometric sensing mechanisms, are used to measure the concentration of the analyte of interest by comparing the sample to a known reference standard. Researchers can also extrapolate the absorbance of the unknown sample on a standard curve that has been established by using several known concentrations of the compound of interest or another relevant substance.
Smartphone Biosensors for Spectrometric Applications
The CMOS-based smartphone cameras can be used to capture images of both target and reference samples. Through the use of a designated software program, these images can be further digitalized and separated into different color spaces, depending on the spectrometry assay that is being performed. The amount of the target substance present in the sample can then be measured by comparing the amount of color present in the target sample to that which is present within the reference samples.
Since surrounding light can influence the quality of the images captured by the cameras, several correction methods can be used to adjust the background light. One of the simplest applications of this technology involves taking pictures of both the target and reference samples when the same surrounding light conditions are present. In addition, plastic reference strips can be used during the image analysis to detect any changes in the white balance.
While these smartphone-based calorimetric applications are very useful, the smartphone-based biosensors can only be used to estimate the amount of analyte in reactions that produce significant changes in color. Therefore, these calorimetry-based sensors may be unsuitable for the analysis of target substances present in extremely low concentrations.
Spectrum-Based Biosensors
Spectrum-based biosensors that have been integrated into the smartphone platform serve as incredible tools capable of measuring extremely low concentrations of target analytes. Unlike the calorimetry-based biosensors, which measure light absorbance at a particular wavelength, the spectrum-based biosensors have the potential to measure the intensities of different wavelengths of light. By comparing the spectrum of the reference substance to that of the target substance, researchers can accurately detect and measure analytes, even when present in extremely low concentrations. While the spectrum-based biosensors are very sensitive to analytes, environmental conditions, such as temperature and humidity, could influence the precision of the spectrum. To eradicate these potential influences, high sensitivity chips that enlarge the changes and construct complex algorithms can be incorporated into these spectrum-based biosensors.
Sources and Further Reading
- Geng, Z., Zhang, X., Fan, Z., Lv, X., Su, Y., & Chen, H. (2017). Recent Progress in Optical Biosensors Based on Smartphone Platforms. Sensors 17; 2449. DOI: 10.3390/s17112449.
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