Breast cancer, the second most common type of cancer worldwide, is a complex disease that, despite screening programs and various treatment protocols, has an increasing incidence rate and high mortality rate [1–4]. Therefore, breast cancer is called the modern epidemic of the 21st century [5]. Like other cancers, breast cancer early diagnosis, significantly increases the probability of treatment success and decreases the mortality rate [6]. Mammography is currently the most widely used breast cancer screening method; however, its false positive rate is high and imposes a high economic cost annually [6]. Accordingly, alternative detection methods more capable of differentiating malignant and benign breast lesions are needed [6]. Blood circulating biomarkers such as cancer antigen 27–29 (CA27-29) and carcinoembryonic antigen (CEA) do not have enough sensitivity for early detection [6]. Due to their involvement in many fundamental cellular processes to tumorigenesis, tissue-specific expression, and their release in body fluids, miRNAs have raised much interest in cancer research as circulating biomarker candidates [7–9]. They are small, single-stranded, non-coding RNAs containing 18 to 25 nucleotides in length [9, 10]. Several studies confirmed the role of miRNA-155 (miR-155) in breast cancer and introduced it as a potential diagnostic and prognostic biomarker [11, 12].
Conventional methods for studying miRNAs, such as RNA sequencing methods, microarray analysis, northern blot, and quantitative real-time polymerase chain reaction, are powerful techniques with good repeatability. Still, they have limitations such as expensive equipment, poor portability, and the long-time detection procedure. These restrictions prevent point-of-care applications of these methods [7, 13]. Therefore, portable, cheaper, faster, and more sensitive methods for application, especially in resource-limited areas, are necessary [7]. Sensor devices were developed to meet these needs, but despite their faster response and portable nature, they could not provide sufficient detection sensitivity. Progress in nanomaterials and their employment in biosensor fabrication because of their size-depending properties and comparable size with biomolecules helped design and develop more sensitive and specific biosensors [14, 15]. The technical issues, research-level technology, and the high cost of nanomaterials-based biosensors are important drawbacks that limit their commercial applications [15]. Colorimetric methods have high commercialization potential among different biosensing techniques due to their simplicity, cost-effectiveness, and no need for complicated instruments [15, 16].
Gold nanoparticles (Au NPs) are commonly utilized nanomaterials in colorimetric sensing due to their unique optical and chemical properties. Au NPs have a high extinction coefficient and adjustable localized surface plasmon resonance (LSPR), and their surfaces are easily modified [16–19]. Au NPs’ LSPR peak location and intensity depend on the size, shape, and the surrounding environment’s dielectric properties. Since the LSPR peak of the Au NPs solution is around 520 nanometers of wavelength, Au NPs’ aqueous solution has a red wine color [16]. Au NP aggregation gives rise to electric dipole-dipole interactions and interparticle LSPR coupling, changing the SPR absorption and color from red to purple/blue/pale pink [18, 20]. Among different Au NP-based colorimetric detection strategies, cross-linking and non-cross-linking aggregations are the most common approaches. In cross-linking, it is noteworthy that the aggregation introduced by Mirkin’s team, a DNA oligonucleotide target, acted as a cross-linker between two types of Au NPs modified by complementary oligonucleotides. Therefore, a network of Au NPs-hybridized oligonucleotides was formed. In this strategy, cross-linkers cause the particles to come closer together, and the color of the solution changes from red to purple [18]. This strategy was the fundamental idea for several Au NP-based colorimetric studies. In the non-cross-linking process, bare or unmodified Au NPs play the role. The bases of the single-stranded DNA (ssDNA) can interact with citrate-capped Au NPs and attach to the particle. Due to the negative charge of the DNA, Au NPs electrostatically stabilized. Complementary DNA strands hybridize with the attached ssDNAs and form a duplex with low affinity to citrate-capped Au NPs, which cannot adequately stabilize Au NPs at an appropriate salt concentration [18]. The second strategy has been employed in some colorimetric biosensors [21].
This study introduces a novel optical method to detect miR-155, a breast cancer biomarker. For designing the biosensing method, the non-cross-linking strategy was inspiring. However, two oppositely charged Au NPs were used, whose interactions were dependent on the presence of the target miR-155. Positively charged PEI-Au NP can destabilize the negatively charged capture probe-modified Cit-Au NPs, leading to aggregation and color change. In the presence of the miR-155, it hybridized with an immobilized capture probe. Positively charged PEI-Au NPs cannot destabilize the negatively charged double-stranded-Cit-Au NPs. Therefore, aggregation and color change don’t occur. This efficient biosensor represents a sensitive colorimetric biosensor for detecting a miRNA model, miR-155.