Green Synthesis of Silver Nanoparticles from Desert Plant Extracts from the Qassim Region for Optical Sensing Applications: A Comprehensive Review

Background: Desert flora and secondary metabolites

An area of dry and semi-dry steppe spans much of the Kingdom of Saudi Arabia, characterized by low and variable precipitation, temperature fluctuations, and vast stretches of sand, which together drive interactions within natural vegetation.^[1-5] The region of Qassim within the country’s central portion represents the archetypal Saharo-Arabian region, where naturally occurring flora are adversely affected by abiotic stresses such as drought, salinity, and large temperature fluctuations between day and night.^[6,7] Yet even Qassim boasts a variety of xerophyte and halophyte species that provide crucial ecosystem services, such as sand stabilization, wind-erosion prevention, biodiversity facilitation, and forage and herbal resources for human use.^[8-10]

From a chemical and biochemical perspective, researchers recognize wild desert plants as valuable sources of secondary metabolites like phenolic acids, flavonoids, tannins, saponins, and alkaloids.^[11,12] The primary active ingredients generally impart substantial antioxidant, antimicrobial, and anti-inflammatory activities, as well as pharmaceutical and nutraceutical value.^[13,14] However, many of these secondary metabolites possess reducing and chelating abilities for the green synthesis of metal nanoparticles and nanoclusters. Therefore, secondary metabolites serve as a point of intersection among phytomedicine, analytical chemistry, and nanotechnology, thereby adding value to the naturally occurring vegetation of Qassim.^[15,16]

Metal nanoclusters in analytical chemistry

Optical properties and sensing principles of AgNCs

Phytochemical composition of the annual species

Recent phytochemical investigations on the seeds, flowers, and aerial parts of the annual taxon from Qassim and neighboring regions have revealed a diverse secondary metabolite profile with pronounced redox activity. Quantitative metabolomic studies report total phenolic contents typically in the range of 40-120 mg gallic acid equivalents per gram of dry material, depending on extraction solvent and plant organ, confirming a substantial pool of redox-active constituents. In addition to phenolics and flavonoids, detailed analyses of the floral parts indicate the presence of both essential and non-essential amino acids, supporting its nutritional value and highlighting the potential for amino acid–based scaffolding in nanocluster formation. Isolation work has also revealed constituents with antioxidant and anti-hyperlipidemic properties, many of which possess reducing potentials compatible with metal ion reduction pathways.

Antioxidant and reducing behavior of the annual species

The antioxidant behavior of extracts from this annual plant has been characterized by using several standard assays, demonstrating strong electron-donating and radical-scavenging activities. 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical-scavenging measurements typically yield IC₅₀ values of 15-45 μg mL^-1, while 2,2-azino-bis-3-ethylbenzothiazoline-6-sulphonic acid (ABTS) assays confirm moderate to strong scavenging capacity, consistent with a dense phenolic/flavonoid matrix. Ferric reducing antioxidant power (FRAP) evaluations indicate substantial reducing capacity, suggesting that these extracts can donate electrons to redox processes that can convert silver ions into metallic silver within nanoclusters, thereby enabling plant-mediated synthesis of luminescent silver nanostructures without the need for harsh chemical reductants.

Phenolic and flavonoid composition of the perennial shrub

The perennial shrub from the Qassim desert flora exhibits a wealthy phenolic and flavonoid profile that often rivals or exceeds that of many commonly studied medicinal plants. Studies employing Folin-Ciocalteu and colorimetric methods report total phenolic contents typically between 50-180 mg gallic acid equivalents per gram of dry extract, together with flavonoid levels in the range of 20-80 mg quercetin equivalents per gram. Within this phenolic fraction, several subclasses of phytochemicals, including phenolic acids such as caffeic and chlorogenic acids, catechin-type compounds, and more complex polyphenolic structures, are identified, which contributes to its overall antioxidant and metal-interacting properties. Such compositional complexity provides multiple functional groups that can coordinate metal ions and stabilize nascent nanostructures.

Comparative antioxidant and chelating properties

Comparative evaluations of extracts from the annual and perennial Qassim taxa reveal broadly similar antioxidant potencies with some quantitative differences depending on assay conditions. DPPH radical-scavenging IC₅₀ values for both fall within 10-50 μg mL^-1, with the perennial shrub occasionally showing slightly more vigorous activity in specific experimental setups. Both plants also exhibit measurable metal-chelating capacity, although their selectivity profiles toward different metal ions can differ, suggesting that they may generate nanoclusters with distinct coordination environments and surface chemistries. Such differential chelation behavior is especially relevant for sensing applications, as it may influence the selectivity and responsiveness of AgNCs toward analytes.

Implications for silver nanocluster synthesis

The combined phytochemical attributes of these Qassim plants create multiple parallel pathways for green synthesis of AgNCs. The co-occurrence of phenolics, flavonoids, and amino acids provides redundant reducing systems that help ensure robust, reproducible nucleation and growth of metallic nanoclusters even under mild conditions. Following reduction, many of these same biomolecules remain associated with the nanocluster surface, acting as natural capping and stabilizing ligands that help prevent aggregation and confer colloidal stability. Because these ligands are themselves bioactive, the resulting AgNCs can inherit antioxidant or antimicrobial features, potentially enhancing their performance in biomedical or environmental applications. Moreover, optical sensing platforms can exploit the intrinsic environmental sensitivity and binding preferences of plant-derived ligands, which may impart analyte selectivity, enabling fluorescence quenching or enhancement mechanisms.^[56-58]

Synthetic approaches and methodologies

Optimization of synthesis parameters

Characterization of green-synthesized AgNCs

Sensing mechanisms and principles

Electron and energy transfer pathways

In an electron-transfer mechanism, the analyte, often a metal ion or another electron-accepting molecule, accepts an electron from the excited silver nanocluster, thereby returning the system to the ground state and dissipating the excitation energy as heat rather than emitting light. This pathway is exceptionally efficient for hard Lewis acids such as Fe(III), Cu(II), and Hg(II), which strongly quench AgNC fluorescence by accepting electrons. When the emission band of the nanocluster overlaps with an analyte absorption band, non-radiative energy transfer, commonly known as Förster resonance energy transfer (FRET), can occur, further decreasing the observed fluorescence intensity.

Spectroscopic methods

Surface-enhanced Raman scattering exploits the strong local electromagnetic fields generated in the vicinity of MNCs to amplify Raman signals from nearby molecules, often by enhancement factors of 10⁴-10⁶, depending on cluster size, morphology, and aggregation state. Silver and gold nanoclusters, therefore, serve as powerful SERS-active substrates, enabling trace detection of organic molecules and environmental contaminants while simultaneously yielding rich molecular structural information through characteristic vibrational fingerprints that can support multiplexed analyte identification. Time-resolved fluorescence measurements provide complementary insight into the excited-state dynamics and sensing mechanisms of AgNCs. Typical fluorescence lifetimes for these MNCs fall in the approximate range of 10-100 ns, with values governed by core size, ligand composition, and local environment, and time-resolved data are particularly informative for distinguishing static quenching, where ground-state complexes lead to non-exponential decay, from dynamic quenching, where collisional processes in the excited state produce predominantly exponential decay behavior. Such analyses help disentangle overlapping quenching pathways and refine mechanistic models for analyte-induced fluorescence changes in sensing applications.^[76-81]

Electrochemical characterization

Electrochemical methods, especially cyclic voltammetry, complement optical techniques by probing the redox properties of AgNCs and their interactions with analytes at electrode interfaces. Size-dependent redox potentials of nanoclusters can be determined from cyclic voltammograms, providing information on their ease of oxidation or reduction and on electron-transfer kinetics, which are crucial for designing systems that couple optical readout with redox-active sensing pathways. Building on this knowledge, electrochemical sensors incorporating AgNCs as electrode modifiers or redox mediators have been developed for detecting diverse analytes, leveraging the high conductivity, catalytic activity, and rich redox behavior of nanocluster-modified surfaces to improve sensitivity and lower detection limits relative to bare electrodes.^[82-87]

Antimicrobial mechanisms and efficacy

AgNCs exhibit broad-spectrum antimicrobial activity via multiple mechanisms that target key cellular structures and processes in microorganisms. Controlled dissolution of silver from the nanocluster surface releases Ag⁺ ions, which disrupt bacterial cell wall and membrane integrity, increase permeability, and ultimately compromise cell viability, while concurrently generated reactive oxygen species under aerobic or illuminated conditions cause oxidative damage to lipids, proteins, and nucleic acids. In parallel, silver species and reactive oxygen species (ROS) can denature essential enzymes and structural proteins and interact directly with microbial DNA, inhibiting replication and transcription and driving cells toward irreversible damage and death. Experimental studies on silver nanomaterials, including NPs and preliminary nanocluster systems derived from the same plant sources, have demonstrated activity against clinically relevant bacteria such as Staphylococcus aureus, Escherichia coli, Bacillus subtilis, and Pseudomonas aeruginosa, as well as pathogenic fungi including Candida albicans, Aspergillus flavus, and Trichoderma viride. The observed antimicrobial efficacy arises from the combined action of the metallic silver core and the bioactive plant-derived ligands that cap the nanoclusters, with the latter often contributing intrinsic antibacterial or antifungal effects that synergize with the silver-based mechanisms.^[91-93]

Antioxidant properties and biomedical relevance

The protecting ligands surrounding plant-mediated AgNCs are typically rich in phenolic compounds and flavonoids, which endow the nanocluster conjugates with pronounced antioxidant activity in addition to their optical and antimicrobial functions. These phytochemicals can directly scavenge free radicals by donating electrons or hydrogen atoms, chelate transition metal ions that would otherwise catalyze radical formation via Fenton-type reactions, and, in some biological contexts, may contribute to the upregulation of endogenous antioxidant enzymes, thereby reinforcing cellular defense systems. As a result, plant-stabilized AgNCs have the potential to offer combined benefits in biomedical settings, acting simultaneously as fluorescent probes for sensing or imaging and as protective agents that counter oxidative stress associated with inflammation, infection, or environmental insults. This convergence of sensing, antimicrobial, and antioxidant properties positions green-synthesized AgNCs as promising candidates for next-generation theranostic platforms, wound dressings, and antimicrobial coatings, where both pathogen detection and reactive species mitigation are required.^[94-96]

Environmental monitoring and water quality

In water quality assessment, metal ion contamination and excess nutrients or organic pollutants pose major environmental and public health concerns that require sensitive, selective analytical tools. Green-synthesized AgNCs can be tailored to detect heavy metals such as Fe(III), Cu²⁺, Hg²⁺, Pb²⁺, As(III/V), and Cr(VI), as well as nutrient species like phosphate and nitrate, and specific organic contaminants including pesticides and pharmaceuticals, by exploiting specific fluorescence quenching or enhancement responses. A key advantage of these systems is their operational simplicity, enabling either visual readout or straightforward fluorescence measurement without the need for complex, high-end instrumentation, while still achieving nanomolar detection limits that are compatible with regulatory guidelines and offering good specificity, cost-effectiveness, and sustainability due to inexpensive reagents, simple equipment, and biodegradable components.^[97,98] Paper-based silver nanocluster sensors enable point-of-care environmental testing, which allows distributed monitoring at or near the sampling site without reliance on centralized laboratories. These platforms often require minimal or no sample pretreatment, allowing direct analysis of water samples and providing semiquantitative or fully quantitative information through color or fluorescence intensity comparison by eye or via portable fluorescence readers, making them suitable for routine field surveys and rapid screening campaigns.

Sustainability and global goals

The green synthesis approach for AgNCs aligns closely with broader sustainability frameworks and international development priorities. Their use in water quality monitoring directly supports Sustainable Development Goal (SDG) 6, Clean Water and Sanitation, by enabling optical detection of contaminants that threaten safe drinking water supplies. By relying on renewable plant materials and producing mainly benign, biodegradable byproducts, these methods advance SDG 12, Responsible Consumption and Production. At the same time, eliminating toxic reagents and reducing energy-intensive steps contribute to SDG 13, Climate Action, by lowering carbon and pollution footprints. Furthermore, when desert or underutilized plants are used sustainably, the approach can reinforce SDG 15, Life on Land, by promoting biodiversity conservation and providing income-generating opportunities in arid regions. From a circular economic perspective, the green synthesis of AgNCs from desert plant biomass exemplifies resource-efficient, low-waste manufacturing principles. The process leverages abundant, renewable plant resources instead of relying on energy-intensive, petrochemical-based synthesis routes. It typically operates in aqueous media without toxic organic solvents or hazardous byproducts, thereby minimizing waste generation and environmental burden. Plant-derived protecting ligands are inherently biodegradable, thereby reducing the long-term environmental persistence of organic components. At the same time, the value-added utilization of locally available flora can support rural economies by creating new supply chains based on sustainable harvesting, processing, and nanomaterial production that integrate ecological stewardship with economic development.^[99,100]

Standardization, stability, and selectivity challenges

Standardization and reproducibility remain major obstacles because plant-mediated systems inherently suffer from variability in phytochemical composition driven by seasonal, environmental, geographic, and processing differences, such as harvesting time, drying procedures, and storage conditions of the biomass. Extraction conditions, including solvent polarity, solid-to-liquid ratio, temperature, and duration, further modulate the profile and concentration of active phytochemicals, and in the absence of universally accepted reference standards and validated analytical protocols for normalizing these parameters, batch-to-batch differences in nanocluster size, fluorescence intensity, and stability are inevitable, becoming even more pronounced when processes are transferred from small laboratory volumes to larger preparative or pilot-scale reactors. Nanocluster stability and storage pose additional difficulties, since ultrasmall silver cores possess high surface energy and can undergo oxidation, dissolution, or Ostwald ripening, particularly when protecting ligands are partially lost or degraded during handling and long-term storage. As a result, fluorescence intensity may gradually diminish due to photobleaching under repeated illumination, thermal degradation at elevated temperatures, or quenching by dissolved oxygen and other reactive species, thereby constraining practical shelf life and complicating distribution and field use. Most current formulations require storage as aqueous dispersions at low temperature (typically around 4°C) and still show stability on the order of weeks to a few months; this situation highlights the need for more robust stabilizing matrices, such as polymer encapsulation, silica or biopolymer shells, and dry-state formulations obtained by lyophilization or spray drying with appropriate cryo- and lyoprotectants. Selectivity and interference management are equally critical challenges, particularly in complex environments, such as food or biological samples, where multiple ions and organic components coexist and can simultaneously modulate nanocluster fluorescence. Multi-ion interference can lead to non-specific quenching or enhancement, obscuring the signal from the target analyte. At the same time, matrix factors such as pH, ionic strength, natural organic matter, and proteins can alter nanocluster aggregation state, surface charge, and local microenvironment, thereby affecting both sensitivity and apparent selectivity. To overcome these issues, improved strategies are needed, including ligand engineering to introduce recognition motifs and the use of masking agents that selectively bind and suppress common interferents. These sensor arrays capture multidimensional response patterns across multiple analytes, and chemometric or machine-learning algorithms can deconvolute overlapping fluorescence responses to obtain quantitative analyte information.

Future directions in synthesis and sensor development

Future research on synthesis optimization should prioritize high-throughput and statistically designed screening frameworks that systematically explore the multidimensional parameter space of plant extract concentration, silver precursor level, pH, temperature, reaction time, and irradiation mode to identify robust operating windows with minimal sensitivity to small fluctuations. Within this context, rational ligand design—where plant-derived ligands are selectively derivatized, fractionated, or combined with well-defined synthetic molecules, offers an attractive pathway to tune surface charge, hydrophilicity, and binding affinity while preserving core aspects of sustainability, especially in hybrid systems that couple the green reducing capacity of natural extracts with the precision and predictability of engineered chelators or polymers.

On the sensing side, next-generation AgNC-based platforms will likely emphasize multiplexing capabilities for simultaneous detection of several analytes via spectral demultiplexing, ratiometric readouts, or microarray-style sensor formats that exploit slightly different nanocluster compositions or ligand shells to encode distinct response fingerprints. Another priority is the development of reversible and regenerable probes that can bind and release analytes without significant degradation, thereby enabling multiple measurement cycles and greatly reducing the cost per analysis; such systems may rely on reversible coordination chemistry, competitive binding strategies, or stimulus-responsive ligands that can be reset by pH or ionic-strength changes. Integration with compact, portable optical readers such as smartphone-based fluorescence detectors, low-cost LED-photodiode modules, or fiber-optic probes will be essential for field deployment in environmental monitoring, food safety, and on-site industrial quality control. At the same time, translation to biomedical applications will require rigorous analytical validation, biocompatibility assessment, and compliance with regulatory frameworks governing diagnostic devices and injectable or implantable materials.

Scale-up, industrialization, and fundamental studies

At the industrial level, realizing the full potential of green-synthesized AgNCs will depend on developing automated, digitally controlled synthesis systems that maintain precise control over mixing, temperature, irradiation, and reaction time, thereby minimizing human-induced variability and ensuring reproducible product quality at higher throughputs. Implementation of robust, rapid quality-control protocols that combine spectroscopic fingerprints, particle-size analysis, and perhaps simple colorimetric tests will be necessary to monitor key attributes, such as nanocluster size distribution, fluorescence intensity, and stability, for every batch. In contrast, life-cycle assessment and detailed techno-economic analysis will help benchmark these processes against conventional routes and justify investment from industrial stakeholders. Engagement with regulatory agencies at early stages will also be important for defining acceptable specifications, safety testing requirements, and labeling standards for nanocluster-based sensors and devices intended for environmental, food, or clinical markets.

Fundamental studies remain indispensable for deepening the understanding of silver nanocluster behavior and guiding the rational design of improved systems. Mechanistic investigations that integrate time-resolved spectroscopy, single-particle methods, and theoretical modeling can elucidate the precise pathways of fluorescence quenching and enhancement, including the roles of electron transfer, energy transfer, and structural reorganization within the ligand shell. Detailed structure–function studies that determine exact silver atom composition, three-dimensional arrangements of atoms, and the spatial distribution of ligands around the metal core will enable direct correlation between specific structural motifs and observed optical or sensing properties, potentially allowing predictive design of clusters with tailored emission color, quantum yield, and analyte responsiveness. In parallel, systematic studies of biointeractions, covering cellular uptake, subcellular localization, immunological responses, and in vivo distribution, are necessary to ensure safe and effective biomedical use. At the same time, environmental fate assessments examining persistence, biodegradation pathways, and ecotoxicity in soil and aquatic systems will help ensure that green synthesis of AgNCs translates into truly sustainable and responsible nanotechnology throughout their entire life cycle.

A rigorous assessment of greenness is essential to objectively evaluate plant-mediated synthesis of AgNCs and their analytical applications, beyond qualitative descriptions of “eco-friendly” or “sustainable” methods. Recent green analytical chemistry metrics such as AGREE (Analytical GREEnness), MoGAPI and ComplexMoGAPI, AGSA (Analytical Green Star Area), and CaFRI (Carbon Footprint Reduction Index) provide quantitative or semi-quantitative scores and pictograms that integrate multiple criteria, including reagent toxicity, solvent volume, energy consumption, waste generation, sample preparation, and analytical throughput. In the context of desert-plant-derived AgNCs, these tools could be used to compare different green synthesis routes (microwave-assisted, UV-assisted, room-temperature) and sensing formats (solution vs. paper-based), thereby enabling ranking of procedures based on their environmental impact and operational performance. A schematic figure summarizing the main features and output of AGREE, MoGAPI/ComplexMoGAPI, AGSA, and CaFRI could visually guide readers in selecting or designing AgNC-based methods that are not only analytically robust but also demonstrably greener.