Dyes as Labels in Biosensing

*Hu Li, Yuanyuan Han, Haiyan Zhao, Hassan Jafri and Bo Tian*

### **Abstract**

Investigation and evaluation of dyes play a vital role in the process of introduction novel labels and their corresponding sensing methods, which signify opportunities for the development of biosensors. This chapter introduces applications of various dyes as labels in biosensing. Bio-recognition molecules with dyes transduce biological information into measurable optical, electrochemical, magnetic or other kinds of signals for quantification. The dyes used in this field were summarized and reviewed according to their signal types, namely colorimetric, fluorescent and electrochemical. Some dyes can transduce signals between multiple physical signals. For some most important dyes, detailed descriptions were given focused on their unique properties, labeling methods and sensing mechanisms.

**Keywords:** biosensors, labels, micro- and nano-particles, optical dyes, charge-transfer complexes

#### **1. Introduction**

Applications of, e.g., clinical diagnosis, drug development, environmental research, security and defense, require self-contained rapid analytical platforms to get rid of tedious operation processes and long turn-around times. In 1960s, applications of enzymes explored a new way in analytical chemistry to obtain specific, sensitive and ease-of-use assays. At the same time, ion selective electrodes have been developed for rapid non-reagental analysis of inorganic ions. In 1962, the concept of enzyme transducer was proposed and sooner the device was developed [1]. Following this idea of enzyme electrode, the first enzyme electrode-based glucose meter was commercialized in 1975. After that, self-contained analytical platforms based on different principles were developed including thermistor, [2] optical fiber, [3] piezoelectric crystal detector, [4] surface plasmon resonance, [5] etc. Today, such analytical platforms are regarded as biosensors. A biosensor is a self-contained integrated device that is capable of providing specific quantitative or semi-quantitative analytical information using a biological recognition element (biochemical receptor) which is in direct spatial contact with a transducer element [6]. Today, biosensors have been applied to a wide variety of analytical problems, e.g., medicine, environmental research, food control, and process industry [7].

Biosensors can be divided into two groups based on involving a labeling process during the detection or not. A label is a foreign molecule that is chemically or temporarily attached to the target (i.e., the molecule of interest) through a labeling process to detect molecular presence or activity. In the above-mentioned early

**Figure 1.**

*Scheme of typical (a) label-free and (b) label-based biosensors.*

period of the biosensor history, typical biosensing processes were usually realized by measuring the transduced mechanical, electrical, or optical signals without any labels, as illustrated in **Figure 1a**. Such label-free biosensors can provide direct information without complicated sample preparation steps. In contrast, label-based biosensors utilizing additional operation processes for higher signal-to-noise ratios and for a broader range of sensing/transducer systems. Conventional biosensing labels are optical molecules or radioactive elements borrowed from bio-analytical systems such as gel electrophoresis and enzyme linked immunosorbent assays. A typical strategy of sandwich assay for antibody-based detection is shown in **Figure 1b**.

Due to the fast development of nanotechnology and material science, nanomaterials are widely adopted as biosensing labels, many of which are dyes in senses of optics or electrochemistry. The special size of these nanomaterial dyes provides unique properties that greatly improve the performances of relevant biosensors. In this chapter, we focus on dyes used as biosensing labels and discuss their properties, applications and how they improve the biosensing properties.

## **2. Dyes in colorimetric biosensors**

Colorimetry technique is a practical and direct analytical method to determine the concentration of colored analyte depending on the color change in solution. The colorimetric biosensing strategy based on this principle has become one of the most popular and important strategies due to its simplicity, visualization, low cost and non-destruction. Based on the strategies of signal generation, colorimetric assays can mainly be divided into two groups, i.e., assays based on enzymes for chromogenic reactions, and assays based on colored labels. Herein, we focus on the later group in which dyes play a crucial role.

#### **2.1 Gold nanoparticles**

As a successful example to employ nanomaterial and nanotechnology to solve the biological problems, colloidal gold has been introduced in the biosensing fields more than 20 years with lots of amazing works being reported. Colorimetric biosensors based on gold nanoparticles (GNPs) have greatly developed in both scientific study and commercial applications.

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**Figure 2.**

*Dyes as Labels in Biosensing*

*2.1.1 Sensing mechanism*

*DOI: http://dx.doi.org/10.5772/intechopen.96540*

solution transferring from red to blue.

**Surface plasmon resonance.** Various GNP-based colorimetric biosensors were built depending on the same principle, i.e., surface plasmon resonance (SPR). SPR is a prominent spectroscopic feature that results in an intense and sharp absorption band in the visible range of noble metal nanoparticles that have an adequate density of free electrons. Localized SPR is an effect that the electron cloud of the nanoparticle sense and start to oscillate at the same frequency as the electromagnetic field of the incident light (**Figure 2a**). During this process the incident light is scattered and converted into heat, both leading to the intensity attenuation of the incident light. Meanwhile, localized SPR produce an electric field on the nanomaterial surface, which can be utilized for labeling several kinds of biosensors. The SPR-induced color is determined by several factors including the size, shape, modified ligands and aggregation state. In colorimetric biosensors, the aggregation state of GNPs is interested. As shown in **Figure 2b**, in bioanalytical assays, the combination of bio-receptors (or targets) labeled with GNPs can induce the isolated GNP assembly as well as the interparticle coupling of the surface plasmon, resulting in the color of

**Surface-enhanced Raman spectroscopy.** Based on the SPR effect, GNPs have also been used in other types of biosensors, e.g., surface-enhanced Raman spectroscopy (SERS) based sensing. SERS as a versatile finger-print vibrational technology has been widely utilized in analytical chemistry, [8] electrochemistry, [9] and media diagnostics. [10] However, the mechanism of SERS is still

*(a) Localized SPR band formation. Red and gray circles represent negative and positive electron clouds,* 

*respectively. (b) GNP agglomeration leads to the shift of SPR absorption band.*
