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. 2023 Jun 8;15(2):81–87. doi: 10.4103/jpbs.jpbs_646_22

3-Aminophenylboronic Acid Conjugation on Responsive Polymer and Gold Nanoparticles for Qualitative Bacterial Detection

Erindyah Retno Wikantyasning 1,✉, Muhammad Da’i 1, Zakky Cholisoh 1, Ummi Kalsum 2
PMCID: PMC10353662  PMID: 37469647

ABSTRACT

Background:

Because of their sensitive and selective responses to a wide variety of analytes, colorimetric sensors have gained widespread acceptance in recent years. Gold nanoparticles (AuNPs) are widely employed in visual sensor strategies due to their high stability and ease of use. Combining AuNPs with a responsive polymer can result in distinct surface plasmon resonance (SPR) changes that can be utilized as colorimetric biosensors.

Objectives:

The purpose of this research is to develop a colorimetric-based sensor through the utilization of the optical properties of gold nanoparticles (AuNPs) crosslinked with pH-responsive polymers poly (acrylic acid) (PAA) conjugated to 3-aminophenyl boronic acid (APBA).

Methods:

The polymer (PAA) was synthesized via RAFT polymerization. The inversed Turkevic method was used to produce AuNPs, which were subsequently used in a self-assembly process using poly (acrylic acid)-aminophenyl boronic acid (PAA-APBA) to create the self-assembled AuNPs-APBA-PAA. The particle size, zeta potential, and reversibility of the polymer-modified gold nanoparticles were determined using a transmission electron microscope (TEM), a particle size analyzer (PSA), and an Ultraviolet-Visible spectrophotometer (UV-Vis spectrophotometer). Visual, UV-Vis spectrophotometer and TEM observations confirmed the system’s ability to identify bacteria. Statistical analysis was performed using a one-way analysis of variance using Excel software.

Results:

Using UV-Vis spectrophotometry, the particle size of AuNPs was determined to be 25.7 nm, and the maximum absorbance occurred at 530 nm. AuNPs PAA APBA colloid exhibited an absorbance maximum of 532 nm, a zeta potential of -41.53, and a pH transition point between 4 and 5. At E. coli concentrations of 4.5 x 107 CFU/mL, the color of the system sensors changed from red to blue after 15 hours of incubation, whereas at S. aureus concentrations of 1.2 x 109 CFU/mL, the color changed to purple immediately after mixing. The TEM confirmed that the detection mechanism is based on the boronate-polyol bonding of saccharides on the outer membranes of Escherichia coli and Staphylococcus aureus.

Conclusions:

The use of APBA in conjunction with pH-responsive PAA polymers containing AuNPs to detect E. coli and S. aureus bacteria induces a maximum wavelength transition, followed by a color change from red to blue. By the process of de-swelling of the responsive polymer, which induces the aggregation of the AuNPs, the established sensor system is able to alter the color. The conjugated polymer and gold nanoparticle-based sensor system demonstrated a promising method for bacterial detection.

KEYWORDS: Bacteria, boronic acid, colorimetric sensors, gold nanoparticles, sensitive polymer

INTRODUCTION

Bacterial infectious diseases account for roughly one-third of all deaths worldwide. Accurate and timely detection of bacterial pathogens can result in immediate and effective measures to prevent or control outbreaks of the pathogens. Many of the current methods for identifying pathogenic bacteria, however, require at least 18 hours between sampling and results. This lengthy response time necessitates the development of rapid and reliable methods for detecting bacteria in environmental, food, and water samples.[1,2]

Nanotechnology is now playing an important role in the development of biosensors. Nanotechnology-based biosensors are being developed for use in the health industry to protect against pathogenic contamination.[3] Because of their high stability and ease of use, gold nanoparticles (AuNPs) are widely used in visual sensor strategies. An optical signal and color transition can be produced by combining metal nanoparticles and a responsive polymer. Surface plasmon resonance (SPR), a unique property of metal nanoparticles, can show different changes in SPR by increasing particle size, composition, distance, or surrounding media, followed by visible color changes.[4,5]

Colorimetric sensors based on nanoparticles and pH-responsive polymers have received a lot of attention. Previous research reported a pH-responsive polymer poly (4-vinyl pyridine) functionalized with graphene oxide and AuNPs to induce absorption and detection of negatively charged analytes at low pH, allowing it to be used as a sensor.[6] In addition, AuNPs can be used as sensors for the detection of Escherichia coli bacteria by conjugating cysteine with polymers that are pH-responsive poly (acrylic acid).[7]

Colorimetric detection with AuNPs colloids can be accomplished through the conjugate binding on polymers triggered by a target to increase sensor sensitivity. The ability of phenylboronic acids to form covalent bonds with diol derivatives such as polysaccharides, dopamine, sugar, nucleosides, antibodies, and glycoproteins is well known.[8] Many bacterial surfaces contain diol molecules and boronic acids containing lipopolysaccharides (LPS), which have been used as identification molecules for bacterial detection. The majority of the boronic acid can be absorbed and aggregated on the surface of bacterial cells. AuNPs-conjugated APBA can be absorbed on the surface of E. coli at a maximum wavelength of 652 nm and increases with increasing pH changes, but decreases at pH more than 5, and the optimal pH value for absorption is 4.5.[9,10] The isoelectric point value of E. coli is 4.4.[11] Another study reported that APBA can be conjugated with pH-responsive polymers by using EDC/NHS reagents.[12] The conjugation of boronic acid against poly (lysine) and alginate, which coat AuNPs, can be exploited to detect diol groups from glucose. The sensor system can detect glucose with a sensitivity of 0.5 μg/mL, where a poly (lysine) bond occurs with the phenylboronic acid group through boronate-diol interactions.[8]

We investigated the use of colorimetric sensors based on AuNPs and pH-responsive polymers, poly (acrylic acid), conjugated with 3-aminophenyl boronic acid (APBA), to detect E. coli and Staphylococcus aureus bacteria.

MATERIALS AND METHODS

1. Materials

All of the materials were purchased from Sigma-Aldrich: hydrogen tetrachloroaurate (III) hydrate, acrylic acid, tribasic sodium citrate, tetrahydrofuran, hydrochloric acid, azobisisobutyronitrile (12 wt. % in acetone), 3-aminophenyl boronic acid, and sodium hydroxide. All materials were used as purchased.

2. Synthesis of Gold Nanoparticles (AuNPs)

As previously stated,[12] AuNPs were synthesized via chemical reduction. In 24 mL of aqua pro injection, 1% sodium citrate (0.18 mM) was dissolved. Then, using a stirrer, 1 mL of 6,5 mM HAuCl4.xH2O was added to the boiling sodium citrate solution while stirring at 500 r/min. The solution turned bright red, and stirring was continued for 15 minutes or until the color was stable. To characterize the optical properties and morphology of AuNPs, UV-Vis spectrophotometry (Genesys 10S) and Transmission Electron Microscopy (JEOL JEM-1400) were used.

3. Synthesis of APBA-conjugated PAA

The polymer (PAA) was synthesized using the previously synthesized the reversible addition-fragmentation chain (RAFT) agents[7] via the RAFT polymerization method. The polymer was then dialyzed against water for 2 days and lyophilized for 24 hours. 1H-Nuclear Magnetic Resonance (NMR) spectroscopy was utilized for the characterization of the polymer (Jeol Resonance 400 MHz). PAA-APBA was synthesized using the method previously described. Around 15 mg of PAA was dissolved in Phosphate Buffer Saline (PBS) buffer with a pH of 7.4 (5 mL; 10 mM). The PAA solution is then placed in an ice bath. While stirring for 1 hour, EDC (25.76 mM; 20 mg) and Sulfo-NHS (9.21 mM; 10 mg) were added to the solution. APBA (25.8 mM; 20 mg) was then added to the solution, which was stirred for 2 hours in an ice bath before being left at room temperature for 22 hours. The resulting solution was centrifuged for 5 minutes at 5000 rpm. The obtained supernatant was mixed with 15 mL of absolute ethanol. A few minutes were allowed to pass before the mixture was centrifuged at 10,000 rpm for 5 minutes. The resultant PAA-APBA was washed three times with absolute ethanol and then vacuum-dried.[13] Fourier transform infrared (FT-IR) spectroscopy confirmed the successful conjugation of APBA onto PAA (ATR Perkin Elmer).

4. Preparation of sensor system of AuNPs-PAA-APBA

Around 25–125 μL of a 1% w/v PAA-APBA solution were added to 1 mL of the AuNPs solution. The solution was then characterized with a UV-Vis spectrophotometer (Genesys 10) for its visible spectra, pH transition point, and color shift when exposed to different pHs. Particle Size Analyzer was also used to measure the zeta potential of the combined solution (Horriba Scientific SZ 100).

5. Qualitative detection of E. coli and S. aureus bacteria in vitro

Bacterial concentrations were prepared in the range of 9 × 103–4.5 × 108 and 1.2 × 102–1.2 × 108 for E. coli and S. aureus, respectively. The bacteria solution was added to the AuNPs-PAA-APBA solution and incubated at room temperature. Using a UV-Vis spectrophotometer, the visible spectra were recorded as the system’s color changed (Genesys 10), and the binding of AuNPs-PAA-APBA to the surface of bacteria was observed using TEM (JEOL JEM-1400) right after the color change.

RESULTS AND DISCUSSION

1. Synthesis and characterization of AuNPs and conjugated of PAA-APBA

Figure 1 depicts the characteristics of the AuNPs synthesized using the inversed Turkevic method. As the plasmon band is correlated with the size and shape of nanoparticles, UV-Vis spectroscopy has been widely used for the initial characterization of the optical and electrical properties of AuNPs.[12] The red hue of AuNPs was due to their 530 nm SPR band [Figure 1a]. TEM analysis reveals that the reduction of gold chloride solution by trisodium citrate solution yielded spherical AuNPs with average diameters of 25.7 ± 2.68 nm [Figure 1b]. Due to the ease of manufacturing, spherical AuNPs in the size range of 13–20 nm with an absorbance peak around 520 nm have been utilized most frequently in biosensors.[14] In this research, we synthesized bigger particle sizes in order to increase the sensitivity of the colorimetric sensor.

Figure 1.

Figure 1

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(a) The spectra of AuNPs solution showed an SPR peak at 530 nm (b) The image of AuNPS recorded using TEM

Using APBA-conjugated PAA, a sensor was developed for the detection of the bacterial outer membrane. APBA was responsible for binding the diol component of bacterial outer membranes.[13] This study utilized the RAFT method to produce a pH-responsive polymer from PAA. This polymer has thiol groups on both sides, which, due to their high affinity for the surface of noble metal nanoparticles, can be used as a capping agent. Figure 2 presents the result of 1H-NMR of PAA, which revealed that polymers peak (δ) at 2.5–3 ppm while monomers peak (δ) at 5.8–6.3 ppm.[15]

Figure 2.

Figure 2

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1H-NMR of poly (acrylic acid) in CDCl2

PAA-APBA was produced by conjugating APBA and PAA with EDC/NHS in pH 7.4 PBS buffer saline. Figure 3 depicts the successful conjugation of APBA to PAA as determined via FT-IR. Strong bands at 3307.18 cm-1 and 1554.18 cm-1 in the FTIR spectrum of PAA-APBA demonstrated the formation of an amide bond. The –B (OH)2 band at 1338.21 cm-1 and the para-substituted benzene ring proton at 796.20 cm-1 in the PAA spectrum and 797.54 cm-1 in the PAA-APBA spectrum confirmed the presence of APBA in PAA-APBA.[16]

Figure 3.

Figure 3

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FT-IR spectra of PAA and PAA-APBA

2. Optical Properties and Characterization of AuNPs-PAA-APBA

The AuNPs-PAA-APBA solution was created by combining 1 mL of AuNPs colloids with a 1% polymer PAA-APBA. Due to the affinity of the thiol group (-SH) in PAA, AuNPs crosslinked to PAA-APBA were formed by self-assembly in the trithiocarbonate groups on both sides of the polymer.[17] System solution exhibits a property, which is the solution’s color change in response to changes in pH. In an acidic state, the solution appeared blue; in an alkaline state, the color changed to red. The color change was clarified by the swelling and deswelling phenomenon of PAA, which can cause the aggregation and disaggregation of AuNP in response to environmental conditions.[18]

Boronic acids have been widely used in the field of biomaterials due to their ability to bind with biologically significant 1,2- and 1,3-diols, such as saccharides and peptidoglycans, or with polyols to prepare a complex covalent or sensitive system. APBA is an amino-substituted aryl boronic acid that binds to the diol group in a bacterial saccharide. The presence of OH groups in the APBA conjugate influences the increasing negative charge in the Au-PAA-APBA system.[19,20] The negative charge of Au-PAA-APBA can interact with the outer membrane of bacteria by forming bonds. As shown in Table 1, AuNPs-PAA-APBA was characterized by measuring the zeta potential of each system.

Table 1.

Zeta potential of the different systems

System Zeta Potential (mV)
AuNPs -31.40±4.51
Au-PAA -31.53±0.21
Au-PAA-APBA -41.53±0.96
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The system solutions for AuNPs-PAA-APBA were synthesized. Figure 4 shows the spectra of the synthesized solution. The absorbance maximum of the PAA-APBA spectrum crosslinked by AuNPs in solution was observed at 532 nm.

Figure 4.

Figure 4

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Spectra of the different systems

The color transition behavior of AuNPs-PAA-APBA was observed with a visible spectrometer and the naked eye [Figure 5]. The advantage of the colorimetric approach is that its results can be detected qualitatively or semi-qualitatively with the naked eye, which is ideal for point-of-care testing.[21] The polymer’s pH transition point is between pH 4 and 5. As the pH increased, the polymer began unionizing and undergoing a phase change from a swollen to a contracted state, resulting in the aggregation of AuNPs. This transition can be observed as the solution changes from red to blue in color. At pH 12, the AuNPs-PAA-APBA spectrum exhibited a sharp plasmon band with a peak at 539 nm, whereas a decrease in pH resulted in a redshift at 700 nm due to the coupling of the plasmon band in nearby particles. The pKa transition of PAA can explain the observed color transition point at pH 5.[22] Below the pKa of PAA, the average inter-particle distance of AuNPs in solution is several times smaller than the diameter of AuNPs. Because of the SPR of aggregated particles, the solution appeared blue. Upon increasing the pH above the pKa of the PAA, the polymer swells, causing the AuNPs to become more distinguishable from a distance and causing the color to change from blue to red.[23,24]

Figure 5.

Figure 5

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pH Transition Point of AuNPs-PAA-APBA

The reversibility of the AuNPs-PAA-APBA solution was investigated using UV-visible spectroscopy and naked-eye observation at a given pH [Figure 6]. The solution’s color change from red to blue is reversible and repeatable up to five times. These color modifications were a result of the swelling and shrinking of the responsive polymer that crosslinked the AuNPs and modulated the disaggregation of the AuNPs when exposed to varying pH levels.

Figure 6.

Figure 6

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The reversibility of AuNPs-PAA-APBA solution in pH 2 and 12 (5 cycles, three replication)

3. Qualitative Detection of E. coli and S. aureus using Sensor System

The sensors of the AuNPs-PAA-APBA system were then used to detect E. coli and S. aureus bacteria [Figure 7]. At the concentrations of 4.5 × 107; 9 × 107 and 2.25 × 108 CFU/mL of E. coli, the color of the system sensors changed from red to blue after 15 hours of incubation, whereas at the concentration of 12 × 108 CFU/mL of S. aureus, the color changed to purple in 0 hour or immediately after mixing. After incubation with E. coli and S. aureus bacteria, spectrophotometry revealed a 2–5 nm shift in the SPR peaks of the AuNPs-PAA-APBA system [Figure 8]. The color change of the sensor system is a qualitative technique to confirm the presence of bacteria. Previous studies reported that the reaction of thiol groups on the surface of phenylboronic acid-modified AuNPs with target analytes could result in the aggregation of AuNPs accompanied by color changes.[25]

Figure 7.

Figure 7

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Schematic representation of the color change of AuNPs-PAA-APBA in the presence of E. coli and S. aureus bacteria

Figure 8.

Figure 8

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The SPR peaks of the AuNPs-PAA-APBA system after incubation with different concentrations of (a) E. coli and (b) S. aureus

TEM was used to observe the interaction between the AuNPs-PAA-APBA and the bacteria [Figure 9]. The images revealed an accumulation of nanoparticles surrounding the bacteria, which may have caused the solution’s color change. Cell walls exhibit aggregate formation. The color change from red to blue in the sensor system is caused by the aggregation of Au-PAA-APBA on the bacterial cell wall. Boronic acid can bind to different sugars in gram-negative and gram-positive bacteria’s LPS and peptidoglycan.[26]

Figure 9.

Figure 9

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The morphological observations of the AuNPs-PAA-APBA aggregation system for S. aureus bacteria on the magnification of 5000 (a) and 20,000X (b), and E. coli on the magnification of 5000 (c) and 20,000X (d). Aggregation can be seen in cell walls and open walls of bacterial cells

A number of polyols in sugars are essential components of Gram-negative bacteria. LPS are found in the outer membrane of Gram-negative bacteria.[27] LPS consists of a lipid, a polysaccharide composed of O-antigen, and an outer core and inner core joined by a covalent bond. Before the sensor solution can diffuse onto the surface of a microbial cell, the LPS and the outer membrane must be traversed. The O-antigen is bound to the central oligosaccharide and contains the outermost domain of the LPS molecule. Before reaching the plasma membrane of Gram-positive bacteria, the sensor solution must first penetrate peptidoglycan. Peptidoglycan is a polymer composed of sugars and amino acids that forms the cell wall beyond the plasma membrane of the majority of bacteria. Gram-positive bacteria have a significantly thicker peptidoglycan layer than Gram-negative bacteria. In addition, various sugar groups are associated with d-alanine peptides containing positively charged amine groups in S. aureus peptidoglycan, causing boronates to bind more strongly to S. aureus peptidoglycans. The difference in charge between these bacteria and the negatively charged Au-PAA-APBA sensor permits differences in the time detection system. It demonstrates that the system is more sensitive to gram-positive bacteria like S. aureus.[28,29]

CONCLUSIONS

In order to detect E. coli and S. aureus bacteria, APBA is used in conjunction with pH-responsive PAA polymers crosslinked AuNPs to cause a maximum wavelength transition, followed by a color change from red to blue. The established sensor system can modify color through the process of deswelling the responsive polymer, which induces the aggregation of the AuNPs. Conjugated polymer and gold nanoparticle-based sensor systems have shown promise for detecting bacteria.

Financial support and sponsorship

RISTEKDIKTI, the Indonesia Government.

Conflicts of interest

There are no conflicts of interest.

Acknowledgement

The authors acknowledge the support of the Indonesian Ministry of Research, Technology and Higher Education (Grant No. 133.4/A.3-III/LPPM/IV/2020).

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