Laser-scribed graphene nanofiber decorated with oil palm lignin capped silver nanoparticles: a green biosensor

Tuberculosis (TB), caused by Mycobacterium tuberculosis (M. tuberculosis), requires a high level of attention and is one of the most infectious diseases in the air. Present methods of diagnosing TB remain ineffective owing to their low sensitivity and time consumption. In this study, we produced a green graphene nanofiber laser biosensor (LSG-NF) decorated with oil palm lignin-based synthetic silver nanoparticles (AgNPs). The resulting composite morphology was observed by field-emission scanning electron microscopy and transmission electron microscopy, which revealed the effective adaptation of the AgNPs to the LSG-NF surface. The successful attachment of AgNPs and LSG-NFs was also evident from X-ray diffraction and Raman spectroscopy studies. In order to verify the sensing efficiency, a selective DNA sample captured on AgNPs was investigated for specific binding with M.tb target DNA through selective hybridisation and mismatch analysis. Electrochemical impedance studies further confirmed sensitive detection of up to 1 fM, where a detection limit of 10−15 M was obtained by estimating the signal-to-noise ratio (S/N = 3:1) as 3σ. Successful DNA immobilisation and hybridisation was confirmed by the detection of phosphorus and nitrogen peaks based on X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy. The stability and repeatability of the analysis were high. This approach provides an affordable potential sensing system for the determination of M. tuberculosis biomarker and thus provides a new direction in medical diagnosis.


Formation of Silver Nanoparticles (AgNPs)
The formation of the AgNPs preliminarily confirmed by the gradual colour changes of the lignin solution. The colour of the lignin solutions changed from dark brown (Supplementary Figure 1a) to brownish black (Supplementary Figure 1b) immediately after addition of silver nitrate (AgNO3) solution. The change in colour indicate the reduction of the silver ion to AgNPs, which is aided by the lignin. The surface plasmon excitation of the AgNPs causes the colour changes of the solution 1 . This show that aqueous silver ions could be reduced by the lignin extract to produce a stable Ag NPs in water.

FESEM and EDX analysis
FESEM analysis is a form of surface imaging which entirely capable of determining the various particles sizes, distributions, shapes and the nanomaterial morphology.
The interaction between electrons in the beam and the sample results in formation of signal used to obtain the information about the surface topography and composition 2 .
The combination of FESEM and EDX analysis enable to determine the AgNPs powder morphology and the chemical composition. Supplementary Figure 2 shows the presence of AgNPs clearly formed from reduction process facilitated by lignin. The nanoparticles were mostly spherical in shape and below 100 nm in size. They are in agglomerated form due to the drying process from liquid before the analysis. Few individual particles were also observed. The elemental analysis of the AgNPs synthesized confirmed by EDX analysis as shown in Supplementary Figure 3. The spectrum shows a strong peak signal at 3 keV, a typical absorption of metallic AgNPs.
The presence of other elements shows the impurities of the prepared nanoparticles. The size of AgNPs obtained are around 18 nm to 53 nm.

UV-Vis Spectroscopy
The UV-Vis spectrum was observed to monitor the stability and the formation of the AgNPs. The unique optical properties of the AgNPs allowed them to interact with the specific wavelength of light 3 . The wavelength scale was fixed from 300 nm to 600 nm.
The peak area formed between 400-500 nm proportional to with the increases of time

FTIR Spectroscopy
FTIR is the most suitable and simple technique used to determine and identify the active functional group in the lignin that involved in the formation of the AgNPs. As shown in Supplementary Figure 6, the lignin shows a wide band at 3500 to 3200 cm -1 assigned to the OH stretching due to the presence of the alcoholic and phenolic hydroxyl group in the lignin structure. The absorption band occurs at 1760 to 1665 cm -1 due to the stretching vibration of carbonyl compounds. The intense peak at 1190, 1088, 1048 cm -1 originates from methoxy groups. The absorption bands in the region ranging from 900 to 700 cm -1 may due to the deformation vibrations of C-H bond on the benzene ring. These functional group show a decreasing in intensities after the reduction reaction to form AgNPs. This indicate the reduction process aided by these functional group.

Electrochemical Impedance Spectra of AgNPs
The EIS of AgNPs were determined by dropping 10 μL of AgNPs onto a screen-printed circuit electrode (SPCE). Supplementary Figure 9