Highly multiplexed immunoassays could allow convenient screening of hundreds or thousands of protein biomarkers simultaneously in a clinical sample such as serum or plasma, potentially allowing improved diagnostic accuracy and clinical management of many conditions such as autoimmune disorders, infections, and several cancers. Currently, antibody microarray-based tests are limited in part due to cross reactivity from detection antibody reagents. Here we present a strategy that reduces the cross-reactivity between nanoparticle-bound reporter antibodies through the application of ultrasound energy. By this concept, it was possible to achieve a sensitivity 10(3)-fold (5 pg mL(-1)) lower than when no ultrasound was applied (50 ng mL(-1)) for the simultaneous detection of three different antigens. The detection limits and variability achieved with this technique rival those obtained with other types of multiplex sandwich assays.

This thesis describes methods for improving sensitivity in rapid singleplex and multiplex microarray assays. The assays utilize the optical characteristics of colloidal gold nanoparticles for the colorimetric detection of proteins.

Multiplexed detection in sandwich immunoassays is limited by cross-reactivity between different detection antibodies. The cross-reactivity between antibodies can contribute to increased background noise - decreasing the Limit-of-Detection of the assay - or generate false positive signals. Paper I shows improved assay sensitivity in a multiplexed vertical flow assay by the application of ultrasonic energy to the gold nanoparticles functionalized with detection antibodies. The ultrasonication of the antibody conjugated gold nanoparticles resulted in a 10 000 fold increase in sensitivity in a 3-plex assay. COMSOL Multiphysics was used to simulate the acoustical energy of the probe used in Paper I for obtaining an indication of the size and direction of the forces acting upon the functionalized gold nanoparticles.

In Paper II, it was studied if different gold nanoparticle conjugation methods and colorimetric signal enhancement of the gold nanoparticle conjugates could influence the sensitivity of a paper-based lateral flow microarray assay, targeting cardiac troponin T for the rapid diagnostics of acute myocardial infarction.

Ultrasonication and signal enhancement of the detection gold nanoparticles has the potential of improving the sensitivity of paper based assays and expanding their potential future applications.

Cardiac troponin T (cTnT) is a biomarker for heart muscle damage such as in acute myocardial infarction (AMI). Its rapid assessment is needed to detect changes in the cTnT levels in blood for a quicker diagnosis of AMI. The sensitivity limit required to detect elevated levels of cTnT is 10 pg/mL, where the levels in the healthy population are 0.5-10 pg/mL. In this paper the detection of cardiac troponin T with a point-of-care lateral flow assay was investigated for the rapid diagnosis of AMI. It was studied by using different gold nanoparticle conjugation methods and colorimetric signal enhancement of detection gold nanoparticle conjugates could increase the sensitivity of a troponin T lateral flow microarray assay. The results indicate the same sensitivity range for the detection with gold nanoparticles functionalized with antibody by two different methods, and that the troponin T sandwich pair used might be essential for achieving a higher sensitivity. The signal enhancement increased the intensity signal of the detected cTnT on the array. The limit of detection of the assay changed from 10 μg/mL to 1 μg/mL for one conjugation method after signal enhancement but remained unchanged at 1 μg/mL for the other method.