Sensor Technology Page 3. 2 LFBs are presented as paper strips, composed of different “pads”,2 as it is shown in the figure above (1). The sample (any liquid, e.g. water, urine or blood) is dropped onto the “sample pad”, made of pure paper (i.e. cellulose) pretreated with different salts and surfactants that control the pH across the strip and help the analytes to flow thought the paper pores, respectively.Then, the sample flows to the conjugate pad (2), normally made of polyester or glass fibbers, where the analyte will be captured by the nanoconjugate (a nanomaterial, responsible of producing the colorimetric signal in the assay, and an antibody that specifically recognises the target analyte).The analyte, together with the nanoconjugate, will flow across the detection pad (3), made of nitrocellulose, and form an “immunosandwich” on the test line (where another antibody will also capture the analyte), which will become visible as the nanomaterial accumulates in the area. In the detection pad also a control line will become visible as the nanoconjugate reaches the line (an antibody immobilized on control line will always recognise the antibody conjugated on the nanoparticles, as a control of the fluidic in the strip).The presence of both test and control lines indicates that the response is “positive” (i.e. the analyte is present in the sample). In the case that there is no analyte in the sample (4), only the control line will be visible, thus “negative” response will be considered. If control line does not appear, it may indicate a failure in the process and the response would be considered “null”, indifferently if the test line appears or not.The last pad, the absorbent pad, is made of cellulose and its function is to absorb the excess of liquid avoiding the flow to stop before the nanoconjugate surpasses the control line. What to know to develop a lateral flow test? Paper (cellulose and its derivates) is the main component of LFBs and understanding how it affects the performance of the test is a key factor when designing the assay. For example, the thickness and width of the sample pad will affect the minimum volume of sample that the user must apply on the test for the sample to flow through all the strip. Also, some specially designed sample pads may help retaining components of the sample matrix which may interfere in the test (e.g. blood cells). On another hand, nitrocellulose porosity rate can be controlled during its production, so knowing the average pore dimension of the material the flow can be controlled.At bigger pore dimension, the flow will be faster, which is good if we want to obtain a response in less than a couple of minutes, but if the pores are smaller, thus the flow being slower, the nanoconjugate will have more time for interacting with the sample, so the sensitivity of the assay could be greater, although the time until one can see the response would be longer. Of course, pore dimension must be also considered regarding the analyte dimension, it is not the same detecting a DNA strain or a small protein than a bacterium or a whole cell.
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