Table 1.

Different SELEX methods that have been developed since aptamer discovery.

SELEX Type	Description	Ref.
Metal-Dependant Aptamers	Enrichment of oligonucleotide library with and without ion salts to generate aptamers that only function in the presence of metal ion salts.	[34]
Crossover-SELEX	First, oligonucleotides are enriched using cell-SELEX. The product of cell-SELEX is then enriched against its purified protein to yield a higher binding affinity. Crossover-SELEX is useful for targets that are rare in their original environment.	[35]
Subtractive SELEX	Selection of aptamers that have the ability to differentiate between two closely related targets (e.g., distinguishing between a normal cell structure and another disease-related one). This is obtained by adding rounds of negative selection against normal cells.	[36]
Conditional SELEX	Selection of aptamers that are affected by the presence of regulatory molecules; aptamer selection is performed in two stages here: The first stage in the presence of regulatory molecules and the second in the absence of regulatory molecules. Only the sequences that successfully bind to the target in either one of the stages but not the other is selected, depending on whether the aptamer is to be used in the presence or absence of regulatory molecules.	[37]
On-chip selection	This is similar to the microarray method. Single and double base variations are introduced using in silico methods to a pre-selected sequence with the highest affinity to its target and then embedded on a surface plasmon resonance (SPR) chip. On-chip selection is useful for aptamer selection against a large number of targets.	[38]
Immobilization-free SELEX or GO-SELEX	First, the library is incubated with the target, and graphene oxide (GO) is then added to the mix in order to bind the unbound sequences via p–p stacking.	[39]
Tissue slide-based SELEX	Selection of aptamers against clinical samples. Cancerous tissue is used in the first stage as a target. Then, the tissue is scraped from the slide with the bound sequences. These sequences are then eluted, and counter selection against normal tissue is performed to eliminate shared aptamers.	[40]
Capillary Electrophoresis SELEX (CE-SELEX)	CE-SELEX separates the target bounded from unbounded sequences by the difference in electrophoretic mobility, which is a highly efficient separation method. This method enables the selection of aptamer candidates with high affinity while reducing the selection rounds to 1 to 4 from nearly 20 in conventional SELEX	[41,42]
Microfluidic SELEX (M-SELEX)	Combining traditional SELEX with a microfluidic system. This system contains reagent-loaded micro-lines, a pressurized reagent reservoir manifold, a PCR thermocycler, and actuatable valves for selection and sample routing.	[43]
High-Throughput Sequencing SELEX (HTS-SELEX)	Aptamers are identified through an iterative process of evolutionary selection starting from a random pool containing billions of sequences. The most predominant characteristic of HTS-SELEX is that it firstly allows for sequencing of the library across all the selection rounds. Thus, enriched sequences are visible at a much earlier round, which is more time efficient. Fewer selection rounds also avoid the potential PCR bias caused by over selection.	[44]